Getting to Cernak first could definitely be one of the hard parts. Trading for him could also make sense, assuming his value is being suppressed by possibility of an offer sheet forcing their hand. As for whether he signs an OS, it'd probably be a significant raise over what Tampa could offer, which is probably a cheap bridge deal. He'd have to be pretty loyal to turn down that much money, especially from another contender, but who knows.
Tampa's definitely in a rough situation made worse by their willingness to give out NT/MCs. Palat, Gourde, Johnson, McDonagh, and Coburn all have full no trade clauses. Stamkos has a NMC, so they can't trade him unless he's willing to go. I think they're guaranteed to lose at least one of Cirelli, Sergachev, and Cernak, and very likely two cause there's just nowhere else to cut.