Yes DR signed a new contract but nobody knows what was in it except for the £5m buyout clause. We also know that DC would only give coaching staff rolling one year contracts which makes trying to plan longer term near impossible.
Yes DR's head was turned by Southampton but we don't know if he had any other terms which he thought would allow him to speak to other teams, etc.
I'm sure there are things that he'd do differently if he had his time over but to expect that he'd just stay and work while may or may not get paid on time, has to do all the office admin as well and coaching the team, has no heating in his office over winter, may or may not have his coaching staff next season and have a boss who appears to renege on just about every promise or ignore every suggestion/improvement requested is insanity.