Matt had his medical June 3. I called DOS on June 17 and was told his interview date would be July 19. We received packet 4 confirming this on June 23. (I was glad I had called, it was nice to know). Hope that is helpful.
1. Obviously, you won't qualify to be the only sponsor, but you collecting food stamps should not be a factor if you have a co-sponsor. Your fiance will NOT be eligible to receive food stamps after he arrives in the US, nor will you be able to get the amount of food stamps you receive adjusted upward because you are supporting him. Your financial situation may be a topic of discussion at his visa interview, especially given his age and the fact that it's unlikely he'll be able to find a good paying job after he arrives. They may wonder how someone who collects food stamps will be able to support a 19 year old who will have a tough time finding a job.
2. Your mom will not be sponsoring you. She will only be sponsoring your fiance. Her household size will include herself, your fiance, and anyone she provides support for. What's the story with her "step-children"? Does she support them? Do they live in her house?
A co-sponsor has the same legal obligations that a sponsor has. The only difference is that they are second in line when the government comes looking for reimbursement for any means tested benefits the sponsored immigrant collects. You, as the primary sponsor, would be first in line. Read the contract portion of the I-864. Have your potential co-sponsors read. It spells out in plain language what they would be agreeing to. The I-134 they'll be signing for the consulate doesn't have any real legal teeth, but you WILL need a co-sponsor in order to get him a green card after he comes to the US and you get married. At that time, your co-sponsor will be signing an I-864.
3. The age difference is irrelevant. It matters at some consulates, but not London.
MalloryEvans reacted to Matt&Deb in How long did it take you to get your interview at the US Embassy in London after your medical?