It sounds like you have put a lot of time and energy into dealing with this problem. It does not seem like others (your husband and in-laws) are putting as much energy into finding a resolution. And, they may never, since for whatever reason, they aren't as invested in making the situation better. But, being right doesn't solve the problem. So, do what you need to do to keep yourself from being stressed out by them. To the extent possible, train yourself to let "silly, inappropriate, and antagonizing" comments go. Unless they are threatening you or harming/endangering your family(which from your post doesn't sound like the case), let the rude comments just pass by. When they say such things, tell yourself, "I choose not to respond to this. I won't waste my energy on this. I won't let them ruin my peace." and just don't. Focus on all that is beautiful in your life---like your lovely daughters---they and the things in your life that you love should be given your physical and emotional energy.
Talk to your older daughter and see how she feels, e.g. how much does she really mind giving up her room, if at all? Would she be willing to give up the room for maybe a couple of nights but not a whole week? Maybe on the nights she gives up her room she could be "reimbursed" for that part of her rent. That might motivate her a little more. These are some questions you could pose to her and include her in on the solution.
Someone mentioned the idea of an air mattress (I've seen some at Walgreen's for a decent price, and they often run sales on them). Why not get one, and if it's okay with your daughter, she could use it in your younger daughter's room. Or, you could put it in your younger daughter's room and you and your husband could use it. Or you could put it in the family room or living room and let the grandparents use it. Once you've gotten your daughter's input, ask your husband what he thinks of the options. If he refuses to add anything useful, then you make the final decision about which arrangement will be used. Tell him you've made an exective decision since you want his parents to come and visit but you were the one left to make the arrangements. Once you decide on a plan, tell the grandparents that you want them to continue to come and visit and that this is the best solution you could find given your current situation and still have them stay with you.
Good luck!