Even if by some magic, Covid is either mostly gone by February, or the "magic vaccine" exists, and has been distributed to all affected parties (players, coaches, staff, referees, game-day staff, etc.), I don't see spring football happening. There have already been multiple B1G coaches expressing concern with injuries due to compressing two football seasons in one year with only +/- 3 months between them. If you don't have them on board, it ain't happening (especially considering they are mostly in agreement with playing this fall).
Add in the likeliness of the high-caliber players sitting out for the draft or in fear of getting injured too closely to the NFL season, and rosters could be devoid of star players. Not to mention the logistical problem with playing in Feb/March in cold weather climates. Even if you ignore the Covid stuff, it just seems like there are way too many issues with pulling this off. And what is the payoff? A potential extra "exhibition game" between the winner of the B1G and the PAC12? If there is no payoff of bowl games, CFP, etc. are players willing to give up a year of eligibility to play 6-10 games? If you're a guy like Coney Durr, St. Juste, and you're a senior hoping to make an impression before the draft, that may seem palatable. But I would think everybody that has a redshirt available would be using it to hold out until a full fall season.