Is it a valid strategy to accumulate cards then swap hand?
There is no penalty for calling out "Uno!" when you have more than 1 card. There is also no penalty for calling out another player for not saying "Uno!" if the other player isn't down to one card. The penalty is for when you fail to call "Uno!" while playing your second-to-last card. So you cannot intentionally increase your hand size that way; at best you could intentionally get the penalty once, each time you get down to one card.

You can choose to not play a card on your turn, even if you have a playable card, and draw a card instead. Though if the card you draw is playable, the rules seem to imply that you must play that one.


  If the player has no matches or they choose not to play any of their cards even though they might have a match, they must draw a card from the Draw pile. If that card can be played, play it. 


Purposefully doing this isn't really an issue of following rules or not, as I can't even think of how you would write a rule to prevent this (allow a person to choose not to play, yet forbid it depending on the player's motivations behind doing so). Rather, this is a question of fun and sportsmanship. Uno is not a competitive game designed to be a serious strategy game. It's a lighthearted family game. Doing something like you describe, whether "allowed" by the rules or not, would lead to simply not wanting to play with you anymore. Or, it would lead to everyone choosing that exact strategy, and the entire game coming down to the luck of who draws a "swap hands" card.