You're an angel putting up with all that - I'd have cracked after 6 days. The first time. Not 6 weeks every year for years on end!
Have you ever talked to your dh and said has he ever stopped to consider what would happen if your parents expected to see you, him and the dc every summer for 6 weeks too - what would happen then, what would his parents do? If you weren't paying so much out for them to stay with you all that time, would you be able to visit your parents or have them to visit for longer?
What would happen if they were yapping at you and one day you just lost it and shouted back at them and told them what you really felt?
What would happen if you told them that they would have a much better relationship with the dc if they came over twice a year for 2 weeks each time so that you didn't have to bear such a long stretch with them? I know that 2x2 doesn't add up to the 6 weeks they usually have but that gives you negotiation room to have them 2x3 or 3x2 - you don't want them turning it into 2x4 weeks!
Does you mil cook or is it something she has people to do for her at home? Could you get your dh to ask her to cook his favourite meal to remind him of when he was young at least once a week - and then it would be one less meal that you need to cook.
Have you tried playing PIL bingo - stick every annoying thing they do on a card and cross them off as they happen. When you get a line / certain number, you get yourself a reward (whether this is a box of chocolates, large glass of wine, nice soaky bath or a night away from the PIL an expensive handbag is up to you 
) But it can help, not least as it turns it into a game so you don't take their words and actions as seriously.
What would happen if you started to talk down to your pil and treat them like naughty toddlers? So if they say something nasty, give them a look and give them the classic 'Did you realise that was a mean thing to say' line. Or if they don't want to eat food from the freezer or microwave or aldi or whatever - tell them to stop being so ridiculous, to grow up and that they are guests in your house, they are old enough to know to respect their hosts and this is the way that things are done here. If they don't like it, they can go back home. But if they want to be over here for so long then they need to accept that things will be done the way they are done over here and basically they can like it or lump it.
Counselling with your DH to get him to explore the different cultural expectations you both have sounds an excellent idea, not least to help him see what his own family is missing out on in terms of money and time and other experiences (ie by spending all available summer holiday with them it means they never get a summer holiday with your parents which isn't fair). And then get him to think about if this is a good thing to be showing as an example to your dc, that (especially if you have girls as it seems you are the one missing out at the moment) would he expect them to spend all their time with their PIL and not see you...
I'd also say that you're saying in advance that next year, as you've spent 9 years hosting them every summer, it's your parent's turn and that you are going to spend the summer with your parents. Doesn't matter if you are going to or not. Probably best to warn your parents in advance about this, sounds like they are nice and will be understanding! But something massive that will put a spanner in the works of their plans next year - and it will be very difficult for them to argue against it because if they turn around and say it's their right, then you can just say that no, it's not. That you have rights too. That your marriage is between two cultures (or three if your parents live somewhere completely different) so you're not going to just stick to just one, you're giving your children the blessing of understanding all their cultures. And that if the pil try to be really tricky, say that you can go back to having every other summer once you have had a chance to even up to get the same number of summers as they have done - so they won't get anything for 9 summers. I'm guessing you wouldn't be able to get dh to agree to seeing your parents for 9 summers - but it's worth talking about it in order to him and your pil see exactly what they are demanding from you. And it also makes one summer off from them sound a whole lot more reasonable!
Good luck - I think you'll need it!