Harry and Molly did not enthrall each other and they did not invade each others mind.
They didn't enthrall each other, but I'm pretty sure they "invaded" each other's minds. Harry describes them playing war with each other's minds in GS. Now, technically, they had permission and they didn't actually make any changes to each others' minds or harm each other, so it wasn't literally invading. They didn't enthrall each other, and even if Justin enthralled Elaine with her permission, that was worse, but to me it if falls into the same category as snowballs v. stones.
Because he tried to enthrall Harry as well?
It looks like he was going to try to enthrall Harry as well, but we don't know that for certain and again, Justin could have had misguided "benevolent" reasons for planning to enthrall Harry, such as training/nemesis detection.
Because being enthralled does not help with anything, ask Luccio.
Did Luccio know she was enthralled? The entire White Council is negligent on mental defense training. Maybe if Luccio had training in being enthralled she could have recognized and resisted the influence and La Fortier would still be alive.
Why infect an apprentice? Nemesis is usually more picky than that.
To take control of a potential starborn before she has come into her full power.
Justin underestimated Harry.
True, but irrelevant.
Elaine could escape because Harry killed Justin.
How? If she was still enthralled, she couldn't move until Justin was dead and the spell was broken, and at that point there is a good chance it would have been too late. Either Justin removed the enthrallment, saved her, or Elaine already knew how to open Ways to the NN and was therefore more powerful at that age than we know.
In ghost story Harry tells about Justin and Lea agrees with him. I think that was conclusive enough.
I don't remember that. I thought GS was the book that gave hints that Justin wasn't all bad, but maybe I'm wrong and need to do a reread.
Justin was one of the bad guys. He would not save Elaine if his own life was on the line.
Maybe, and more likely true than not, but there is absolutely room for reasonable doubt.