1) Public transport has been in operation all through out the pandemic, they reduced the train service but there's photo's from the height of the pandemic with those who still had to go to work all rammed into carriages, so I don't think that will be affected or a consideration really.
2) That is definitely a big concern - if they have to isolate for 2 weeks that probably makes it financially non viable. Though there is a high chance they won't have to if they have had the vaccination, but that probably makes it hard to find guests who have had it, that are free to come over, want to do the show and are affordable. Also, many attendees are based overseas and I know many are hesitant to buy tickets as they don't know if they will be allowed to travel either and don't want to buy if they then can't come over and not get their money back.
3) I agree, thought the first dose does give good coverage it is a concern.
I know it's very easy for me to say this as I'm not familiar with the intricacies of running a con and what the concerns are behind the scenes, but it does seem like it would be more sensible for the larger events to hold off for a bit.