I've jumped to reply at the start of page 2 so sorry if I've missed anything.....but
Does this really need to get to 3 pages long! Youre asking will the tiles fall off in a couple of years? To be honest, who the hell knows what will happen, point to he made is IF they do, what can be done!
Bottom line your Builder can not guarantee anything, all he is giving you is words. Mapei won't do anything if they do because you've not used it correctly or followed the instructions/recommendations clearly printed on the side if the tub.
Your builder will no doubt of scarpered, changed trading names because he's been putting peoples toes at risk for 40 years and someone has finally decided to sue him.
You've read the tub, know the score, now stop the muppet and ask him to start again and do it properly. Or get someone professional in to do it.