I'll echo what everyone else has said - don't do it. It sounds like a terrible idea.
Fundamentally, you cannot replace the relationship with her husband and it's not fair to expect you to try to do so. She really needs to try online dating or making new friends if she is lonely. She is not that old. I agree with pp who have said she may expect to be over every night. There are also lots of red flags in terms of manipulative behaviour etc.
I think that, whilst your e-mail is totally fair, it would probably go down very badly. If it were me I would maybe send a slightly briefer version along the lines of the following (which will probably still upset her but it sounds like anything other than giving in would upset her...)
"Hi Mum,
I wanted to talk to you about earlier. [or is it yesterday now?] I love you but you know I think living together will not work for a number of reasons. I know that you have been having a tough time since Dad left but this is not the answer. Amongst other things, I think that living together (even with a separate annex) would put a strain on the relationship between you and me which is not what either of us would want.
I know you are disappointed that I do not think this is a good idea, but I am finding it upsetting to keep discussing it so was hoping we could move on and talk about other things now.
I’ll call you at the weekend,
(Jasnah)"
I think either you or someone probably does need to put to her at some point that if she's not lonely since her marriage broke down, she needs to find some company in terms of either new friends or a new partner, but I wouldn't put it in the e-mail as she'll probably react very badly to it even though it's true.
A tough situation but it sounds like her suggestion wouldn't work for anyone (including her) so stick to your guns and good luck 