You really think Teach can win against both Law & Sengoku even tho he's old ? His crew doesnt really matter unless Oda fucks up powerlevels like he did this chapter with Law huffing from using a little haki against Doc & a rock.
Oda already pulled Rayleigh thing once and he won't repeat the same back to back for teach when teach is getting set for next opponent for Luffy.
How it will be resolved if sengoku really appears is worth seeing but most likely if sengoku really appears then by the time he will, law would be defeated with BB having PGs.