First of all, congrats, because you found it. I have found that many people do not have the patience to do such work. So, I appreciate you.
Most of the validation errors are from the original theme, I think. Anyway, it is not a nice idea to change the theme's original files, as they will be lost when they update the theme.
I have checked many sites, and most of them have some problem or the other. Though, it is advisable to have a clean validation, it is nothing to worry much about unless there are too many errors and WARNINGS.
I think your site is OK. I will recheck one or two aspects. Come back after a few minutes. If your problem is solved, mark the post as resolved after a few minutes.