Kel, not to stick up for anyone or anything, but speaking AS someone who is a MIL, I would like to offer you a fresh perspective, if I may. (Oh, and first of all, God bless you for ALL you do for your GMIL and your family!)
I think your MIL may not necessarily be as wishy-washy as you think as far as whether or not she wants the chickens. Perhaps she agrees with HER mother when GMIL is present and goes along with her just to keep peace. I notice that when you and MIL were alone, she sympathized with you and welcomed your chickens to be there, as well. Can you imagine what it must be like in her shoes, stuck in the middle between her son and his wife who are just starting their lives fresh, and her elderly/blind/cantankerous mother? I would not trade places with her in a million years. In her heart, she probably could care less whether or not you have chickens... but she "sides" with GMIL in GMIL's presence to keep HER from going on and on and being more cantankerous than she already is.
I am not there and do not know the situation 100%, but I am just offering you a possible other perspective to consider. Don't give up just yet on getting your chookies. If you feel you can do it, then do it! Nobody knows your own abilities better than you do, sweetie.