Next time you mash, make sure you have a good crush, your mashing temps are where they need to be throughout the entire mashing process and make sure that the grain and water is mixed thoroughly to ensure that all of the grain is in contact with the liquid. Lastly, pay attention to your sparging technique.
I would always encourage you to take your OG measurement pre-boil (post sparge).
Boiling again will jack up your hops utilization and flavor profile.
Hopefully you know your system well enough to accurately predict boil-off rates, etc.
Yes you should boil anything going into your unfermented wort. You should boil the dme separately in a minimal amount of water because of the hops. Or the easiest thing to do is just let it roll as is and just work on your process for next time.