Will BA/IAG need 14 daily flights from LHR-NYC in the next couple of years? Probably not.
Even if not, they can still use the slot for another destination, different flight number, aircraft type potentially if they wanted to. A slot, put simply, is effectively a landing/take-off 'space' on the runway as part of the airport's declared capacity. They could still in theory cut a NYC flight and use the slot for MCO, for example.
if BA plans to have 80% of its pre-covid schedule back by the new year, it wouldn’t have to lose a single slot in either LHR or LGW.
It doesn't quite work like that, unless I'm misunderstanding your point? The 80/20 rule applies to specific slots, not the entirety of an airline's schedule. For instance lets say BA operates LHR-AMS twice a day on BA123 and BA456. If they failed to operate more than 20% of the BA123 flights across the season, they'd lose historic rights to that slot only. The BA456 would be unaffected.
Also the 80/20 rule is only, at this stage, waived for S20. Currently the W20 slot submissions, which airlines are compiling at the moment, are as usual.
Any excesses can be leased to other airlines - Vueling for example.
Unlikely that would happen. I don't imagine Vueling will be in a state of full growth mode either to open a major LGW base. And the cost involved for transfering BA aircraft to Vueling?- It's virtually a non-starter. On that point, nobody is going to be in agressive growth mode, so holding surplus slots purely to avoid competition getting them or leasing them out in a weak market is an unlikely way forward.