As per the title. I bent my mech, cant find M8100 in stock but i can see an M8120 available. Would M8120 work? Two images below for comparison.
https://bike.shimano.com/en-EU/product/component/deore-xt-m8100/RD-M8120-SGS.html
https://bike.shimano.com/en-EU/product/component/deore-xt-m8100/RD-M8100-SGS.html
Two things to consider. One is chain capacity, how much slack chain will the derailleur cope with, and the second is the angle of the parallelogram which dictates how wide the cassette range can be.
Can you use an 8120 with a single chainring? Yes you can. After all, on a 2x set up, you're only using one right at any given moment.
Can you use an 8120 with a 10-51 cassette? Probably not. You might get lucky, it might just work, but it's not going to run as well as the 8100. Would work perfectly if you use a 10-45 cassette.
After smashing an xt I just switched to slx which is about 20g heavier. Didn't notice any difference in function. Merlin have them in stock.