Ace is always high. It’s the highest card in the deck above the King.

You can but it would render that card spot “dead.” You can continue to play any other cards on that spot but when it comes to scoring the round your opponent would earn one point for you not making your sequence (and possibly another point if they made a proper sequence on their card spot).

Technically no, but two people could be on a team and one person on the other.

No they don’t. Although, if you can manage to make it same suited it will beat an identical sequence that is not same suited. For example: A double sequence of Ace of Clubs and a King of Clubs would beat an Ace of Hearts and a King of Spades.

You can keep swapping (switching) cards as many times as you want in one turn as long as you can place a card that is in sequence of the cards that are already in the card spot you want to place it in. If you place a swapped card in one of the completely blank single card, double or triple card spots then your turn is over. Remember to grab a card from the pick up pile if there are any left ( for 2 player version).