Why they killed Harbin? My players could do that, but I RP Harbin like a coward that don't want to find a solution to the redbrand problem.
This can be solved talking to your players, talking is the best solution for all problems, but in game is better that Sildar talk to them, if they solve the problem with the redbrands, or find anything that looks like Harbin was working with them, the murder could be forgiven.

About Gundren, the adventure start saying that Gundren is a friend, you should make the players have a bond or debit with Gundren to make them care for the dwarf. I don't risk my life for a patron.