I don't see any direct way to remove a friend from suggestion list, But you can unfollow a friend (remain friends) to not get notification and to not see their name in suggestion list at top.
Note that unfollowing a friend will not show their activity on your timeline.
You can unfollow any of your friend at anytime, they will not get notified.
About most of your question, you can't find them if they blocked you, unless you have some kind of list to compare to and find the differences.
how can I tell if someone has blocked me?
It's actually what you mean. You know someone blocked you, but you don't know whom. I'm also not sure if those apps actually work or just try to collect data, never tried ...