When I grew under lights, we would go away twice a year for 2 weeks. We just watered the heck out of them the day before we left and then turned off all but one light per tray. Then as soon as we got back, we watered them thoroughly. Most of the plants faired just fine. What I'd be worried about if I were you would be the mealybugs. Keep that plant far away from the rest and try to get that under control before leaving. At the very least I'd be spraying the heck out of that plant as well as the surrounding plants with horticultural oil mixed with water.

One thing a friend who has well water that is high in minerals does is to make a water run to my hose once in awhile and use half and half for her orchids. That reduces her minerals enough for her orchids and she doesn't have to come off the mountain so often. Instead of going to all RO water, you could try mixing it to make the mineral level lower. All RO water has issues of its own. It is hard to get a consistent pH.

Just becaue you don't think it's "salty" doesn't mean it isn't. Depends on what kind of soap. A "detergent" type body wash thingie may be less of a problem, but "soap" is made by combining alkali with fat. Basic soap may well have sodium hydroxide (lye), or it could be potassium hydroxide, which would be less of a problem. If the soap you use doesn't have an ingredients label, call the manufacturer if you're that interested. They may not be willing to tell you exactly what's in it, but if you push hard enough and are specific, they might be willing to tell you if there are "sodium-based substances" in it. (If you're really lucky, you might get a real answer to the basic question whether they have any idea if it's OK to use gray water from their product on "salt sensitive" plants - orchids are far from the only plants that have a serious problem with sodium.) Other than that, as long as it's not mostly what you're using for watering, I think it'd probably be OK for the "tougher" orchids like Oncidiums, Cattleyas, Phalenopsis, Vandas, etc. More "tds-sensitive" plants like your Zygopetalum, Paphs, Phrags, Miltonia/Miltoniopsis, let alone many of the Pleurothallids, might take it less in stride.... Barring further research, try it on one of your Catts for a month or so - it's unlikely to harm any of the above-mentioned tougher plants, but should give you an idea of what you're working with. I would definitely not rely on it as a primary source of water, and wouldn't use it to mix up fertilizer - without a lot more information, you'd have no idea what fert compounds it might bond with either rendering them less available or conceivably, rendering them "harmful" in the sense of causing a problem - in any of several ways - if not necessarily immediate poor plant health.

They all sound like they can't get too wet, it's hard to overwater mounted orchids. The cloud forest plants probably would not appreciate a 5 day dry period. Sounds like you are on the right track. The other thing you can do, if you have enough sphagnum moss lying around, is to create a wet pad of moss and lay the mounties in it so just the roots are touching the wet moss. Nick