"Also, not sure if this is related by I'd estimate that >95% of the time I'm sitting down my lower back is rounded. Is this common? "
Maybe, but if you are concerned with posture you should stop doing it
"also, even without a barbell, I can't even come close to completing a good morning with proper execution. My back just doesn't bend that far down. "
I didn't think that those movements had a certain bend that you had to get to. Just go til you feel a stretch then come back up? For RDLs this would mean you don't actually set the weight down. Ofc, you could work on flexibility on the side if it is important to you to get it all the way down.