You should push up to champions for the 2,000 gems that you get when first arriving there and the nice loot bonus.
Definitely xbows, they are more expensive generally but mortars effects can be canceled out by a simple heal spell. The mortar has like no dps and the heal spell will counteract its effects while still healing the troops back to full health. I am th10 with like max defenses but still have lv 6 mortars, considering that htey are the most worthless defense in the game, try for wizard towers instead.
Well #1 I think you mean power potion not resource potion, and number 2, it does not, it is still the same level, the blue flame is just to represent that you are using a power potion at that time. It doesn't actually make a difference, considering the fact that the troop gains no additional levels.