In reply to David Coley:
I've only dabbled in the alps but have done a fair bit at that altitude in winter elsewhere so my thoughts;
> 1. Will these be warm enough?
Single night maybe, but take a brush and a system to get the ice off them and keep them from freezing. Reproofing helps a bit. More than single night I'd really think about doubles, especially if you're doing north facing stuff.
> 2. Is it worth pairing them with Berghaus insulated yeti gaiters?
See above. The problem is a) the Nepal's leather holds moisture and the gaiters hold it in, b) heat loss through the sole is a bigger issue and doubles or higher spec boots address this with insulated midsoles. Insulated Yetis will help from snow clogging the laces and a small amount with general warmth, but a short term solution I think.
> 3. Do yeti gaiters even work with step-in crampons. I own an old pair of Grival G14 crampons with a metal bar at the front. So not sure how the bail at the front and back will fit with the sole groove full of gaiter.
They're meant to but a front basket is better. Grivel do an add-on one.
> 4. I also own a pair of plastic double boots, which whilst fine for stomping up a high snowy hill in Nepal, I'm guessing are going to be hard to climb on rock as just so bulky? But am I wrong?
As in Vegas or similar? If in good condition (consider how old and their reputation for material failures) and comfortable they'd work. Many of the Alps hardest routes were done in them after all. Depends on how hard the climbing will be, though they'll be nicer for bivies.
The Nepals will be ok for the climbing, it's the practices of keeping them wearable that's the issue. All the questions I've asked too and answered by getting boots better suited to the task, the extra expense was made up for the first cold night out watching friends hack ice out their laces then stuff the cold boots into their sleeping bags with them. They were sometimes a hindrance in the morning, sorting cold boots when we could have been climbing. Per extra dollar I've got more good climbing in with less screwing about at bivies.