Unless he's clearly not ready, Byfield should be on the club this year, not even a question. Shortened season or not.
If you're a GM sending a player back to junior primarily not to burn a year off his ELC you're doing it wrong.
1) If QB wants to be on the big club and proves he's ready, then sending him back would be a transparent sign that you're more worried about how much you're paying him than what he thinks and wants. Way to get off to bad start with a potential franchise player.
2) The sooner he starts his mentorship under Kopitar the better.
3) If it's all about money, then you'll possibly get a better RFA deal if his 3 ELC years start when he's 18 rather than 19. (Just ask the Maple Leafs re: Marner as one example. That 3rd year cost them a few million AAV when he jumped from 69 points to 94).
Btw, Bettman is pushing for a full season and unless something with Covid goes drastically wrong - which is certainly a very real possibility - you'd be a fool betting against him.