If we can decant the small aircraft off LHR's main runways their resilience / productivity will be improved; this is the logic behind R3 and applies just as well to the NHT option.
No it does not.
Landing on a different runway (even a Polderbaan) and then continuing to the same terminal (or at least complex of terminals) is a completely different concept to landing at another airport.
You can't just build a pop up airport on the cheap. We did this here at CVT, but still got in trouble with planning issues. The airport also opened with the ridiculous situation of having no on-site parking - EVERYONE had to come in by shuttle bus for the first few months. No bus service from the city centre for the first few months either.
All we ever tried to do here was run a no-frills operation - from the ultimate Ryanairportakabin terminal (although Ryanair never popped in, runway length being a key issue). I also note 07/25 is 1684m. How long would the re-aligned runway be?
Are you seriously suggesting that some temporary operation, designed to cater for some A321/738 ops but not others is just going to appear by magic within a year?
BA have had enough difficulty juggling which destinations they can't fit under T5. If they need to move aircraft (and baggage) to operate from another terminal, they can still do so. How many LHR-NHT movements would be needed just for positioning?
Is there a case for a Flybe type operation at NHT - to feed in from the regions but still be closer to London for domestic hops to CAX, NQY etc than SEN and other airports are? Maybe, but might I respectively suggest that should be on a separate NORTHOLT thread (if it doesn't already exist)?