The best way to deal with humidity in the bathroom is to install an exhaust fan. But on to your question -- you are re-painting walls that are already painted, is that right? I don't think plaster walls require anything special -- I use Kilz primer followed by semi-gloss paint. If semi-gloss is too much for you, a satin or pearl finish should be fine. There are also additives you can add to the paint to avoid mildew (ask at paint store). My dad does that since he refuses to use anything except flat finish paint. I go with semi-gloss and no additive needed since it is very scrubbable. If there are mildew spots now, I would spot treat with bleach first. If not, you should not even need primer unless you need it to hide the old color. Plaster is actually superior to drywall IMHO in dealing with moisture. It won't grow mold like drywall. Good luck!

Toggle bolts are fine for this application. Once the head expands inside the wall you just keep some pressure on it while you tighten the screw. Pull the grab slightly toward you while tightening the screw will keep the head end against the inside of the wall and it will not spin.

I know you're worried about how the top coat/finish will look but you should consider what's underneath that, first. Plaster is rarely one coat, almost always two, or even better, three. Each applied separately and left to dry before the next is applied. The different coats are made of slightly different materials as they have different purposes, too. But the first thing to think about is the lath. I assume you are talking about wood lath? You said you have bonding stuff, but that's not going to be sufficient to stick the plaster to the lath. It's really intended to stick plaster to plaster, at the edges of the repair, which except in really small repairs isn't enough to secure the plaster to the wall. Plaster works largely because it is mechanically held to the wall when some of the first (deepest-in) coat is squeezed through the openings in the lath and blobs of it form behind it within the wall cavity. When you tore the old plaaster out, you no doubt saw the plaster material behind the lath, that wasan't sloppy work, it was an essential part of the plaster. Through a chemical curing process as the plaster dries the material on the room side of the lath and the blobs behind are permanently bonded together around and through the lath. Subsequent layers of the plaster are bonded to the deepest layer (and to each other when there are multiple coats) by a reactivation of the chemical reaction as they are applied and then dry. But the strength of the wall is based completely on the integrity of the first coat's attachment to what's behind it. The blobs are called "keys" and in a mechanical way they "lock" the first coat plaster tight to the lath. The strength of this layer comes because a chemical reaction occurs when the plaster dries (cures) slowly enough to become rock-hard and tightly stuck to the rough surface of the lath. Originally the wood lath (like all new wood) had enough intrinsic moisture content that it did not suck the moisture out of the drying plaster prematurely which would interfere with the vital curing process,thereby weakening the plaster. However when you are dealing with old lath (that was original to the wall), which is now almost tinder dry, you can have a problem. One solution is to try and re-wet the lath by spraying it to the run-off point repeatedly over several days just before starting the repair. You can also strengthen the repaired-section's adhesion to the lath and to the wall by attaching over the wood lath a section of expanded metal lath. Cut it to roughly fit the space and screw it to the wall studs. (Don't tighten the screws down so much that the metal lath is distorted and pulled down into the spaces between the lath strips. You want it to be a flat, secure plane lying just on top of the lath.) Do not use nails which can cause additional plaster pieces nearby to crack and come off. Very small repairs (6-8" holes) can get by with just attaching (withwires, or careful stapling) the expanded metal lath to the wooden lath itself, but anything larger needs the metal lath attached to studs. Your first coat of plaster is a rough coat and its pupose is to get good adhesion of the plaster to the wall so it gets troweled onto (and through) the lath pretty firmly. It should remain completely below the surface of the finished wall and there's no need to make it more than nominally flat. It is usually a much coarser mix and slitght roughness on its surface helps subsequent layers stick to it. Only scrape off big lumps which would interfere when the next layers are smoothed on. The first layer is allowed to dry and another one or two coats are added on top to fill the remaining space and bring the repair up to the same plane as the wall. Each will be slightly different in composition; the final one being more fine, and/or in your case more textured. As recommended above I would definitely try as many trials of your finished texturing as necessary before doing your wall. You needn't used multiple coats while you are practising the top coat texturing. Just try the finish samples on scraps of sheet rock over and over until you've got the technique for the final coat down to a satisfactory match. Top coat plaster sets up quite quickly and you want to be able to apply it, work the texture and be done before that happens. Before starting this job I recommend doing some more research on working with plaster. There are instructions in books on old house repair and from time to time in magazines that deal with old houses. You should also know that depending on the age of your house modern plaster available for repairs and orginal plaster are not the same thing. However for a small repair I think I would use modern materials rather than trying to source and work with older formulas. HTH L.

Please don't replace with Drywall! You can save your old plaster and reattach it to the lath beneath without the waste. Plaster has significantly different acoustic properties from drywall and makes the room more "live" feeling. Plus it keeps plaster out of landfills. A complete tear out may cost much much more than repair. A family member of mine just recently paid $8k just to have a bonus room drywalled and insulated. This room was unfinished so there was not demo or trash removal to pay for. You can't buy the craftsmanship that went into your walls today. It hardly exists anymore and, in this day and age, labor costs as much as or more than materials. There are great ways to repair your plaster with a plaster adhesive. This Old House covers the topic, too. Here is a link that might be useful: plaster adhesive