I agree with some of what you're saying, but what Trump said to begin with and what he's spinning now are two different things.Trump is unelectable. Period. Full stop. He is at best a side-show and at worst will split the vote and hand the election to Hillary.
But what he is saying is quite true. John McCain hasn't done a damned thing for veterans or his constituency except support the borders being opened. He was last in his class at Annapolis. He effected little damage on the enemy during his service. He refused to fight back in the general election. He selected a mindless (but very hot) neocon for his running mate. I defy anyone here to cite legislation sponsored or written by John McCain that was positive for the USA.
Being captured and then conducting oneself as McCain did in captivity is honorable and he is a bit of a legend because of it but I would not categorize him as a War Hero.
Alvin York was a war hero. Robin Olds was a war hero. JFK was a war hero. Huge difference.
Yes, McCain hasn't been the greatest politician of all time. Yes, he has made the most of his POW status. Why not? Yes, he ran a mindless campaign for president in 2008, not that it would have made any difference. Dubya was such a colossal screw up there wasn't a republican alive who would have had a chance in 2008.
Finally, on his hero status. The word hero is thrown around loosely these days. Just doing military service doesn't make one a hero. However, the man had a chance to be released early from the POW camp and he turned it down. I don't know many guys who would have done that. Yeah, he's a shit politician, but that one thing gets him pretty high marks in my book and Trump is a complete asshole for saying anything disparaging about it.