I know how hard mixed culture relationships can be. my DH is hindu and his parents live in india, they come over every year during the warmest time in India to stay for 3 months with us.
we pretended to be married when we were first living together, which was no big deal to me but really helped his parents out at home, we then had a quick registry office wedding here when we were ready with just both sets of parents then had a big wedding in india. It made it so much easier for my inlaws to say we were married to their friends and family.
we also have religious pictures up, but only ones that I like, I am not Hindu, DH isn't practicing Hindu. I guess we see them as cultural rather than religious. I am so lucky though as FIL has very good taste and everything they have given is more like a work of art.
As for the cooking situation, I am honestly ready to jump off a motorway bridge by the time they go. It is constant cooking and eating, cooking and eating for 3 months. It is a constant nightmare and completely sexist as FIL does nothing, he doesnt even carry a cup to the kitchen, even if he is walking through the kitchen to get to the downstairs bathroom. MIL does most of the cooking, though I chop and prepare all the food for her several times a day. They do like to try english food once or twice a week. The problem for me is there is no consept of a light lunch, or god forbid skipping a meal. If i'm not hungry i still make myself eat or ww3 breaks out.
I have insisted since the first visit that DH makes drinks and does the dishes, which has FIL grinding his teeth.
They have always been good about speaking english for me and I have learnt a fair bit of bengali now, can understand much more than i let on. Its not long since they went back this time and i asked them to speak mainly bengali as I wanted DS to be exposed to the language as much as possible.
I think that your in-laws are behaving appallingly but from you post it sounds like neither of you will compromise, which you must have known would be necessary when you chose to spend your life with your DH.
You need to sit with your DH and go through all the problems. Make a decision on what you will compromise on and which you wont. If you wand to save the situation you have to give some ground, would a few religious items really be so hard to swallow?
You need to give your DH a really good kick as he is the one who should be supporting you. if they try treating you like a slave make sure DH knows to get up and do the jobs instead of you. It has to come from him for them to get the message on this one if my in-laws are anything to go by. It would have been a deal breaker if my DH had not started to support me and I made sure he knew it.
Sorry for such a long post, it took us years to get to a point where I didn't want to hurt someone an hour after the plane landed.
Best not to mention my public meltdown the time they tried to rename my baby......lets just say they discovered where I had drawn the line that day!