Sleep rock thy brain, And never come mischance between us twain!

I’m sure she means rocking in the sense of rocking a child to sleep, in terms of rock-a-bye baby, in terms of a rocking chair – the sort of rocks that lulls a person to sleep.
But I always hear it as in ROCK! As in Rock n Roll, baby! Rock that brain! Sleep, shake it like a mega-ton stadium show!
Rock thy brain!
Like a whole bank of speakers, like a set of earplugs to make it tolerable, like lights flashing, guitars wailing, arms slamming and a whole lot of strutting.

Nor earth to me give food, nor heaven light, Sport and repose lock from me day and night, To desperation turn my trust and hope, An anchor’s cheer in prison be my scope, Each opposite that blanks the face of joy Meet what I would have well, and it destroy, Both here and hence pursue and lasting strife, If once a widow, ever I be wife!

These are the sorts of oaths most people live to regret. Especially if they break them. I mean, these are some pretty dire consequences just for moving on with your life.
I mean, I could understand an oath like this if you were, say, swearing allegiance to your country in battle. It makes some sense if you need to assure your commanding officer that you’re not going to turn traitor.

But I suppose this is the same thing in the domestic sphere. Men have to swear allegiance to their country and women have to swear allegiance to their husbands. In both cases, the one with little authority is assuring he who has more that he or she is not going to do any betraying.

I mean, we never see a male character have to swear his fidelity to his wife after death. It’s always assumed he’ll remarry and the ladies are constantly assuring them that they should.

That very plot point in David Copperfield, for example, drove me fucking bananas. Oh – the stupid little woman is so sweet and so loyal; She WANTED me to marry the woman she knew I loved all along. She told me so herself. So it makes it all alright!

Anyway – I agree with Gertrude.
This lady is overdoing it.

A second time I kill my husband dead When second husband kisses me in bed.

So dramatic, Queeny!
Killing your husband just with kissing.
But the thing is. A dead husband is already dead. Dead is kind of a one-time thing. You can only kill someone once. And you can only kill someone with actual killing, too, not with kisses (unless you have, like, poison lipstick like they do in spy stories or Doctor Who or comic books – then yes, sure, a poisoned kiss could kill someone, sure. . but. . .)
The only sorts of people that can be killed more than once are a) Gods b) videogame characters c) cats. But not really.
It would be rather amusing, though, to have a husband die, come back to life, he’s walking around, happy as you please and then keels over again as soon as his wife kisses her 2nd husband in bed.
Then he comes back to life again.
Then she kisses again.
He dies again.
She kisses.
He dies.
A loop of rebirth and death.

The instances that seemed marriage move Are base respects of thrift, but none of love.

Is this the 3rd mention of Thrift in this play? I believe it is.
And the 2nd that relates to marriage and thrift. Which makes me suspect these as some of the lines Hamlet has inserted into the Murder of Gonzago.
But why should a second marriage be more thrifty than first?
I can see how marriage in general is a good expense saver. Joining economic forces for two people makes good sense. And when one is lost, I guess the same economics hold true.
It is a very odd thing to say.
Especially as a Queen.
I mean, middle class folks might get married to save money but royalty? They get married for alliances, for politics, for kingdom security, etc – but thrift? I don’t know.
You could make the case that this line is one of Hamlet’s insertions because of the language/idea connection between thrift and marriage but it doesn’t explain what he could be hoping to accomplish with it. Is Royal thrift a different thing than middle class thrift?

None wed the second but who killed the first.

Wormwood. Wormwood.
This line has the remarkable quality of sounding like a saying, as in:

Like they always say, “None wed the second but who killed the first.” Or “A stitch in time saves nine!” And so on.
But of course it is an insane saying.
To suggest that every re-married woman is a murderer is nutsy pants. But the genius of this line is that the sound of it makes it sound like a thing. Like, that old chestnut spousal murder!
Also, it’s chock full of meaning for the wider vision of the play – Hamlet’s response to it would suggest that this might be a line he put in. But why? As far as he knows his father was murdered by his uncle. End of story. No murdering required of his mother. Or perhaps this is a little misdirection – look over here at THIS impossible proposition! But – really, we all know it’s this other thing.
Oh good old husband murdering!
Happens all the time.
Except it almost never happens.
Wife murdering, okay. That’s actually a thing. A third of all murdered women in the USA were murdered by their male partners. 
And when it does happen, that is, when women murder their husbands, it is almost always in self defense after a long period of domestic abuse. None wed the 2nd but who killed the first after a long excruciating period of violence and abuse that finally came to a head and she killed him to survive.
That old chestnut.

In second husband let me be accurst!

Careful what you wish for there, Queenie!
We all know where these sorts of curses lead. Straight to accursed 2nd husband, in this case.
Myself, I’ve learned to never say never.
“I’ll never more to NYC,” I said, before I moved to NYC and stayed for over a decade and a half.
That’s how this sort of language works. If I declare that I’ll be damned before I sink into that kind of poverty again, I’ll be looking at either damnation or poverty or both, before the 2nd act.

Such love must needs be treason in my breast.

On the news quiz show, they mentioned a study in which a large percentage of the women surveyed had back-up husbands in mind. That is, there was some man that they knew that they had visions of turning to should their current marriages not work out.
I guess it turns out that a large percentage of married women had this kind of treason in their breasts. It’s kind of a small treason though, isn’t it? And potentially evolutionarily sound.
Like, if evolution has us partner up for the benefit of children, should we lose said partner, it might be good to have another in mind to help us and our genetic line survive.
But, sure, treason in the breast. Okay. I mean, just because one thinks about a 2nd career as a fireman when one’s singing career doesn’t work out doesn’t mean you’re committing treason against Plan A. It just makes sense.

Where little fears grow great, great love grows there.

This must be why my theatre life fills me with terror. It’s that horrible great love of the goddamn theatre. It is a great motherfucking love and the little fears get bigger everyday. Although not all of the fears are little. The “will I ever be able to make a living doing what I love?” one is actually not so small.
But there are little ones that grow big, too. Can’t think of what they are at the moment. But there are. I’m sure of it.

Where love is great, the littlest doubts are fear.

My first real committed boyfriend was a big fan of a book about relationships which featured a concept called the Upper Limits. It postulated that relationships often followed a roller coaster pattern of going way way up – then falling quickly down because the way way up led people to a place where they felt they couldn’t get any happier.
Couples who are so blissed out in this love state will often find some way to plummet again, for fear of the maximum, of going so high you just run right off the rails of the ride.
They’ll find things wrong, they’ll stumble into quarrels over little things, they’ll get insecure or fearful.
We ran into this problem a lot in that relationship – though, in retrospect, I’m not so sure that the upper limits were quite as high as we thought.