Renting sight unseen is always a risk of it being a scam. Is the rent significantly lower than is common for other rooms in the area?
Not signing onto the main lease is fairly common when renting a room. You are signing a sub-lease, and as long as the people who are offering you that lease are legal tenants, you become a legal tenant just as they are. There is a risk that they've not gotten approval to make sub-leases, but that doesn't make you any less of a tenant... just if the landlord finds out they may decide not to renew to main lease, but your sub-lease still remains in effect until it expires even if the primary tenants leave.
To answer your questions directly:
If they actually live in the building and they signed the sub-lease, they can't stop you from moving in. If they tried to, you can call the police and present your lease. I really doubt that would happen, assuming they legitimately live there.
They wouldn't need to break their lease to add another person, they'd just need permission from the landlord, so it's possible they're lacking that permission; or the landlord may have blessed the sub-lease. The only way you could know is to ask and get the landlord's contact info.
You could ask one of them to Skype or Facetime you and show them walking into the residence from the street. The most surefire way is to actually come to SF to view the place before signing.
You have legal protection if the current tenant(s) sign you sub-lease and are legal tenants themselves. If it's just some scammer who doesn't live there, you have no protection.
When moving somewhere, visit for the weekend and line up a bunch of housing visits to inspect rentals in-person. If anything seems suspicious like they won't show you the inside, just consider it a scam and move on.