As far as this episode, I agree with you on the last two teams not giving up.
I also think the Express Pass was used at the wrong time. Stringing a harp isn't super duper hard (if you know how to thread a needle you should be able to restring a harp), and the watermelons were time consuming but given the bottom row was given a high border, all you had to do was carefully line the mellons where four met at an intersection. This was a clear case where the adrenaline of the race caused them to not think smart. It also cost them the 2 hour penalty for not completing the road block smashing those bottles, and knocked them from 1st to 6th - as Paul clearly said making it a first that a (at least single team) penalized team wasn't eliminated. (I say single because technically that eating task Rob and Amber failed to complete they survived - but only because he was able to get others to agree to not eat in the challenge, saving them).
And I mean really - you'd have to be rushing that challenge too much to break EVERY SINGLE BOTTLE ON a table. They had to have had at least 20-30. None of the others were assessed the penalty, even the last two teams and you know they were in a head-to-head to stave off elimination.
Watch them now go to either a desert area, China mud area or Norway and have to go dig or roll out haybales for a race clue needle in the pile or mud/hay/sand. That honestly is the only time I can see - save possibly a skydiving, bungee or food eating challenge - to use an express pass.