Clean the threads of oil/grease & use a little blue loc tite on them to keep hand guard torqued . You will still be able to remove it, if need be .
There are a lot of good triggers out there . If you want to keep it with a std. type trigger , Spikes Tactical has a enhanced trigger set , that are NIB coated & mine settled it at about 5 1/2 # & is nice & smooth .
ALG Defense also has some nice smooth triggers in stock configuration , the one I have has settled out at about 5 1/4 # & is also coated .
Both triggers are in the $ 60 range, The ALG has two types to choose from .
And of course , there's always the higher dollar ones , which are much better.
Most don't have the problems , you are experiencing with your DPMS . I think they have the largest production numbers of all that pattern of 308AR's & with a lot of them being produced , you get a few that are sub par . Sending it back many times is not a good reference to there customer service, that's for sure .