If I'm a college player that just went through no season in 2020 and I am eligible for the 2021 draft, then I am definitely signing up. I'd rather be a pro-player if something like this happens again than still be in college.
To be NFL draft eligible, you only have to be 3 years removed from your high school graduating class. There potentially being no college season in 2020 will not impact NFL eligibility. It will only impact how the NFL evaluates the players, but the NFL still will want to bring in young and relatively cheap talent if it can in 2021
It most certainly would not be fair to just use the same draft order as in 2020. A weighted lottery is the closest thing to fair as they will get. Base it on the record of the teams previous 3 seasons or something similar. I even think they should do a lottery for each round and the deeper the draft the less they weigh the records.
In my opinion, I think there is a stronger chance of there being an NFL season, but not a college season. Still lots of time to figure things out.