They're pushing for Bernie because of the socialist label. The same label they use for every Democrat. It's watered down. Them preferring Bernie isn't a sign of any future result. Hillary wanted Trump. Clearly didn't work out. Trump can't use his outsider image against Bernie. Bernie has the best record on unions, trade (which Trump admitted privately was the reason he didn't want Bernie as Hillary's VP pick), and doesn't take any money from corporate donors. So yes, Trump has indicated he wants Bernie, yet he's also spent that entire time talking about how the establishment is trying to screw him over. Literally communicating to his dumb base that Bernie has integrity.
This is the type of narrative Bernie can use against Trump
It's not even just Hunter Biden.
1. Biden voted for NAFTA, which doesn't bode well for the midwest.
2. Biden doesn't turn out the youth. The reliable vote is the older voters. They're going to vote blue no matter who. If Bernie is the nominee the older vote is the reliable vote. The youth vote is simply an addition that Biden will miss out on.
3. Biden is terrible in debate
4. Biden can't use the social security attack against Trump because he has advocated for the same thing multiple times.
This doesn't change the fact that Bernie will fight the hardest to enact positive change. Bernie is the only one willing to campaign in whichever state whichever Dem doesn't help enact said positive change. You're relying on Biden to simply work things out without applying political pressure from the masses.
Foreign policy? This is the one area that doesn't have congressional approval, and thus the one influence you don't need them. Bernie is without a doubt better than Biden here. Maybe you don't think it's possible for the president to enact change here. I would disagree, but at the very least you should admit that Bernie is the more likely president to show restrain when wielding this destructive power on brown people.
While I agree that they'll paint every Democratic candidate as a socialist, it'll be particularly effective against Sanders because he will proudly embrace the label. And that's not a problem for him, but it IS a problem for every candidate down ballot in a red or purple state. I don't think it'd be impossible for Bernie to win, but I do think we should exercise caution when giving Republicans exactly what they want. I don't know how worried they'd be if Sanders becomes President since I imagine they'll have a better chance of retaining the Senate and perhaps picking up seats in the House. I do not agree that Sanders has the best policy on trade. We need to move more towards globalization, even if that goes against the current nationalist mood. I also disagree that Trump can't use his outsider image against Bernie. Bernie actually knows how the government works. Trump thinks he can just do whatever he wants. There's nothing more "outsider" than someone who is literally clueless.
In regards to your points:
1. Voting for NAFTA shouldn't be a negative, but it probably will be. The USMCA is basically NAFTA but I'm sure has a lot more support in the midwest because it has a shiny new coat or whatever.
2. There's an assumption that Bernie will get exactly Biden's coalition plus the youth vote. I think that's optimistic. Just because Biden supporters aren't online 24/7 sharing memes about politics does not mean that they're less enthusiastic about voting for him over Sanders. I feel very strongly that Sanders will lose at least some significant percentage of older voters, black voters, and more conservative Democratic voters. In addition, we know how unreliable the youth vote is.
3. and 4. aren't too significant imo. Biden has consistently shown to have strong support among the older population in the primaries. And the debate thing is overblown. Biden did fine in the VP debates. But it won't really matter, because Trump will just mock Biden's stutter or talk about Sanders' "rape essay."
I agree that Sanders will be more vocal about enacting positive change, but I am not quite convinced how effective it will be. Sanders has been yelling about doing this and that for the past few months. What has Biden been vocal about? Not much, but within 48 hours, basically the entire party was making a mad dash to endorse him. It could just be a coincidence and he could be super lucky, but I'm going to guess he has at least some ability to convince others to do something mutually beneficial.
For foreign policy, Sanders is probably better in that particular aspect. I think he would be forced to do things that he'll get chastised over, though. I do think we should just get the hell out of a lot of these countries and drastically tone down our spending on the military, which Sanders is most likely to do.