Lamar can build arm strength in the weight room and if he believes that's a weakness he'll do so. Not real worried about that.
Big receiver and inside linebacker are indeed needed. Just as important is a damn good edge rusher so they don't have to blitz all the time.
They have got a huge amount of cap space to improve a position or two with free agency. They can maybe get another position in the draft but that will be tougher since they will be drafting so low due to their record.
I don't buy it that Harbs is to blame. He was completely behind the re-design of the offense. Committed to it in full. He also kept the team focused through a grueling 12 week winning streak where there were many opportunities for a loss that never happened. I don't think the team was not 'up' for the game, they just didn't know how to deal with adversity if they fell behind. The offense lost its poise a little bit when they fell behind. Roman was too quick to give up on the run I thought too but the guy is still a genius who created the team's overall strategy during the re-build. And the offense is very young at the skill positions and was handed a spanking.
Get some receivers to get open and catch. Build up Lamar's arm. Get a ferocious edge rusher. If they get all that and lose again, then I would start looking at the coach.