Are you trying to use the BGGET to get the tile value to determine what kind of collision is needed? You could use an array containing the id numbers of tiles that collide and once you BGGET a tile, iterate through that array to determine if it's one of them. This would be a kind of manual process though.
The other thing you could do is if you aren't using all of your bg layers, one of them could contain collision data. In the map editor in the smile tool, you could just paint where you want collision. You wouldn't display the layer in your game, just check it with BGGET, and then if the tile is anything other than 0, it's a collision.
The second part of your question, is that your character movement should always get calculated. When the user presses left, you add the left movement velocity to the sprite, but store the old position first. If your character is only moving a pixel at a time, you can just revert to the old position if the new position causes a collision. It's more complicated if you're moving faster than that, but let me know if I've understood the problem correctly.
