He insistently wants to see you. -> He is insistent that he wants to see you.
I will wait until the day you sell it to me
I'll wait until the day you sell it to me (spoken), or, I will wait until the day in which you sell it to me. (Business formal)
If I had known how pretty it was here, I would have come before ->
If I had known how pretty it was, I would have come here before
If I had known how pretty it was here, I would have visited before
("Come before" is an idiom meaning people of the past who have achieved things before, in the tradition of a Hall of Fame. Your sentence was correct, but it sounds off because of that idiom that exists.)
What do you think if we moved into another house? (Perfect!)
I would have felt bad if I hadn't done something about it.
Perfect, but I would say "I would have felt bad if I hadn't done anything about it." Both are fine, but anything is more common in this expression. "Something" is used more in the positive: "Let's do something!" "Will you do something for me? (Asking a request for help)"
While anything is used in the negative. "He wasn't doing anything." "I don't want to do anything this weekend except relax." "They didn't do anything about the problem."