Yes, I would double it for a 9×13-inch pan. That keeps the mascarpone layers thick enough and gives the graham crackers enough moisture to soften properly overnight.

I probably wouldn’t try lifting the whole thing out of a 9×13 though. It becomes a pretty large soft dessert once chilled. I’d still line the pan with parchment, but plan to serve it straight from the dish
instead of transferring it to a platter.

Yes, I think ladyfingers would be really good here. I would use the crisp cookie-style ones instead of the soft sponge cake version though, since the soft ones would probably disappear too much once they sit overnight in the cream and lemon curd. The crisp ones should soften into layers closer to a strawberry lemon tiramisu.

Digestive biscuits are probably the closest overall substitute for graham crackers because they soften overnight similarly and still keep a little structure between the cream layers.