dot and dab is a common method used by non tilers as it saves on addy and is easier to adjust when fixing
adhesive manufacturers reccomend min 70% coverage but most tilers work on 90% min
porcelain are dense and therefore heavier than biscuit and need a good purchase to the substrate
another advantage of 90% coverage is when you press the tile in you push the air out creating suction holding the tile where you placed it
youve been given sound advice on here
with care you will be able to remove all the tiles and clean them ready for reuse
score through the painted surfaces and then prime, dont just prime as the primer wont penetrate the paint and the paint may seperate from the wall dropping your tiles
good luck mate
its difficult when mates or family help out as you dont want to fall out with them but you really cant leave this and the sooner they are off the softer the addy
mike
ps i gave up smoking last week after 40 yrs , i can reccomend as ive got a new lease of life................still climbing the walls though