Well it's not deciding really but working out what means more to you. Many people prefer photo's because it's visual proof that they met the guest and they love that they can look at the photo and remember the moment. Others don't like photo's as much and prefer to get autographs as you can have a slightly longer interaction with the guest. It really is just about which you prefer.
Personally I love getting autographs especially if I'm doing a cast multi signed item as I can look at my posters, have them framed and put up and I think they look awesome. I do get photo's but usually it has to be with people I'm really hyped about or it's related to my absolute favourite show/film as otherwise they don't mean a lot to me.
I don't talk to the guests much as I'm not a great conversationalist and I'm not as into film as many people who attend these events are, so I can't wax lyrical with them about insightful stuff on their works. But you don't need to really, with the bigger names you probably won't get time to do that anyway. So I just usually say something like I'm glad they came ect, or something might rarely strike me that leads to a conversation - I met Zach Quinto at LFCC 2019 and got him on a Star Trek poster but actually told him I loved him in Hannibal so we had a brief chat about that which was nice.
Deciding days is hard to discuss because it depends on too many factors - which days you can go, which days the guests are there, how many you want to meet and what else you want to do. If the guests you want are there Friday, that's a good day because it's the quietest of the 3 days so can be a little calmer, but if the guests you want are only there 1 day you have to see them that day. If you are looking for advice on how to do the event again it's hard because it depends on what you want, but a good way to start might be to go on a Friday and get a feel for it, how it all works, maybe see a couple of guests and you can see what you prefer between the auto's and the photo's. Then if you find you like it and want to do more you can try Saturday and/or Sunday. Saturday is the busiest day though and at LFCC it''s very busy.
I'd not advise telling the guests about disabilities unless it's relevant to the conversation as it can put them under undue pressure, but if the situation requires it then do. If you are feeling really anxious and having trouble coping it would be fine to say that you have anxiety and to give you a minute or whatever you need. Also speak to the crew if you need help on the day, if you need a quiet place and it's fine to let them know why as they will understand then. Showmasters do have an Access pass for people who need extra help that might be worth looking into and applying for if you feel it would help you and you qualify. Here's the Access info for Spring, but it does the same thing at the other events as well https://www.londoncomicconspring.com/index.php/accessibility