To have a new pickup that has already been tore down to the extent this one has is BS and depending on what is really wrong they may not be done yet. They need to make this right, what that means is up to the buyer but simply saying it is fixed and sending you on your way wouldn't be enough for me. New pickup, long extended warranty, money back, whatever but to pay for a new pickup with this amount of tear down with less than 1k miles is not right.
As to the gas engine pickup value, I have come out very well on both a 2011 2500 Chevy with a 6.0 and a 2015 3500 with a 6.0. I was worried but the dealer was happy to trade gave me what I felt they were worth and neither pickup was on the lot more than month. Gas trucks will resale, remember when figuring trade value it is done off percentage of purchase price. The diesels hold their value but you pay a lot more as well. Not everyone wants a diesel or is willing to pay the premium for them and that is reflected well in this forum.