I'm on Solmar's first trip of the season (late Oct. - early Nov). Don't remember the exact dates.
Currents - there is some but never had any up or down. Generally speaking, current is always strongest at the surface so it is of course conceivable to be swept away a bit (seen it happen in Cabo Pearce and Roca Partida) - but nothing like Cocos or Galapagos.
There have been a couple of instances - at the Boiler and at Cabo Pearce - where swimming down to the site was a bit of a struggle - but the crew ties a line to the rocks so that you can pull yourself down hand over hand and not suck half your tank away just going down.
Never had any really strong currents at depth - the kind that can rip your mask or reg off or that would force you to quickly find shelter in the rocks. Just keep in mind that what I am telling you is based on just three trips.
Depths are what you choose them to be within the recreational limits. Solmar dive guides are pretty strict about not going below 40m and might make you sit out a dive if you do. Normally, the deeper dives are done in the 25-35m range. Some have bottoms, some have deep bottoms, and some do not. For example, the Canyon is good for just about everything: I've seen hammerheads, silvertips, and even a tiger there. If diving for sharks, you'd dive it at 25-30m. But if there are mantas and they are playful, you might spend the whole dive at <20m, so really no need to go deep on some dives.
The afternoon dives might be in 15-20m max even tho they can be dived much deeper. In this case, the main limiting factor isn't so much where the bottom is or how much air you have but your no-deco time.
As for the need for air redundancy, if you are talking about a pony, that may be easier to accommodate. Again, best thing is to ask.
If you get swept away, chances are that you would come up and stay on the surface anyway. Granted, if it's rough, it might be a bit of a struggle but I've never seen really rough conditions.
If you are low on air and away from the boat, the panga will pick you up. If you are low on air near the boat and still need to do your safety stop, Solmar hangs two tanks with regs off the stern at 5 meters.
As I said, I've never been on the Nautilus so I can't speak for what they do, but enough people in this forum have dove that boat and liked it enough for me to safely say that they're probably very capable and professional as well.