Oh dear OP. I really feel for you. My IL's are seriously interfering too. As with yours and theowlworriors they do it under the guise of concern/helping/family but it really is because they don't see you as adults in your own right, just as extensions of themselves so they don't understand boundaries.
I know loads of people of my generation (35) with parents like this. Want to know how much they earn, go on hol together etc. But when i point out they never would have done this with their parents they make out it was because their parents generation didn't love them as much or that it's okay because they are much more knowledgeable/fun/interested etc and their parents were stuffy - but hey they are great so it's totally different (which is just a bollox excuse to be nosy/take over).
Also i can imagine the fake hand wringing of 'oh poor invesufferedenough, look she's not earning anything' under the guise of concern, but in fact it is just pure gossiping. I also hope this doesn't become a 'family' topic of conversation, still pretending to be 'we are all concerned for you'. When i was made redundant IL's kept doing that, but all they really wanted to know was what payout i got and for us to tell them how much our bills were.
Without digressing too much, from my personal experience, i think the baby boomer generation are extraordinarily entitled and cannot see that they can't just do what they want when they want. This may not be the case for you, but as i say, this is the situation all of my friends are in at the moment. I think it's because now we need help longer (living at home, uni etc) that the strings don't get cut in time and the relationships are just left to carry on.
My IL's want to know how much we pay for everything then try desperately to find it cheaper somewhere else. They never do, what they find is a variation, which they think is fine, but it always lacks the exact reason we chose the original thing/service. Then we spend ages justifying why we want the one we want, despite it being a bit more expensive (sometimes we are talking a tenner difference). I have had to send 3 emails explaining why we chose our buggy rather than the 3 second hand ones they bought without consulting us. And don't even get me started on car insurance or our carpet!
Anyway, what to do OP. If i were you I wouldn't go away when they visited. Just an opportunity for them to steam roller DH. I would ensure i was there and challenge them every time they overstepped the mark. I would make sure DH spoke to them about the post and i think i would mention it too. I would also stress that in future they must ask BEFORE booking visits. Then stick to it, if they do it again say they can't come.
Anyway, i've rambled on enough. Good luck OP but stick to your guns. You might as well set the boundaries now. You've already half ripped the plaster off, so you might as well go the whole way!