Thanks for summarizing everything. Now to answer your questions (and if I remember the spoilers here right)
2. Novel version made him find out the truth when his own father (despite not liking MC) did his own private investigation to all of MCs "crimes". It wasn't that hard to find actually since corrupted Sandra while talented in seduction and manipulating men (thanks to her stealing beauty) is apparently terrible at hiding her tracks. I mean, as someone here already said, even during MCs condemnation event, a few people wanted to step out but couldn't (since I assume it'd be like arguing with the current CP. Remember, 1st Prince doesn't have a good backing). Basically, Evans just fell in love at first sight to a pitiful crying maiden and went biased with her actions and not do a lick of investigation
4.I assume its the same excuse as her granpa, related to her powers going haywire (I mean, earlier spoilers mention granpa didn't have to go so far as to lead to MCs home banishment but he had to since both princes were incapable of protecting her, so better for her to become a commoner.
Honestly, while this has already been answered, it still bothers me that their was NO POINT for her granpa to be so strict