Those ratings are uncomparable, since humans only play humans and computers only play computers. It's probably way lower than 5%.

Tactics: https://en.lichess.org/training Video library: https://en.lichess.org/video

No, there are positions where computers are known to make the wrong decisions. Computers use opening books instead of evaluating the moves themselves because they don't evaluate openings well.

I've used both, lichess' free is better than chess.com's premium.

ICC is basically dead, even top players are leaving. Takes forever to get a game.