Do intrusive thoughts and inappropriate sayings invalidate the prayer? And should one withdraw from the congregational prayer if his prayer becomes invalid? And do clothes stained with a small amount of urine droplets transfer impurity?

Chattering to oneself does not invalidate the prayer, even if frequent. Whoever's ritual purity is broken during prayer must interrupt it, perform ablution anew, and restart the prayer. A garment soiled with urine does not transmit impurity to another garment unless one of the two garments is damp or wet.