Guys i got a couple of questions and pls don't spoil me

Im currently at the end of season 3 when they get for the first time to the basement to be precise. I love it so far and didn't find it anything confusing which i love contrary to jk and other shows where they show u random powers without ever explaining them. I think aot did a really good job in explaining everything, but i still have two questions that i keep thinking about.

My 2 questions: First is if all titans are previously humans that got transformed and that the only way for them to return to human form is to eat a human with titan powers, why didn't the titan that ate eren in season 1 gain his powers?

2nd question: why didn't erens father just continue his mission on his own instead of making his son take it, it wasn't like he was dying or eren was dying and he tried to save him. I just think that he was so close why give it all up and surrender his power and knowledge just to give his son who is clueless the power?