How do I pop bubbles before they turn to coins?

When you select the bubble at the bottom of the screen it will give you a description of the item, and will also show a pin and the cost of unlocking it in rubies. Click on the pin to burst the bubble - you'll be asked to confirm, then it will generate a coin without waiting for it to time out and turn into a coin.

Yes you can, select the bubble, then there should be an option to break the bubble, and you'll get one coin for it.

Ideally you should save your coins and merge them until you get them to the top level before redeeming them on the game board.