Well, yeah you are probably right that they probably had some sort of plan B but I do believe them when they say that Spidey was their plan A and not their plan B, as some have speculated before. In other words, he was always in the script. If they wanted him in the script, of course it makes sense they are going to plan as if he was going to be in so they can start planning for the best cameo they can have rather than wait and then squeeze him in half assed later.
They probably just made sure to plan the script to scrap all of his scenes if the deal didn't go through, like have most of his scenes fighting with the Avengers need CG or have most Tom Holland scenes be with only part of the cast so it could be shot separately. That also fits with the lack of Tom Holland being seen on sets and even if the deal did go through pretty late (which it seems like it was kind of pushing it as is), they could have still planned for Tom Holland shots to be done in the scheduled reshoots.
