The best that anyone can tell you is that it depends on what things look like when you open it up. That's a fair amount of water over a period of time that you are talking about. I would want to open things up enough that I could see if there was standing water / saturated wood anywhere. Just getting wet isn't a big deal... wood gets wet during construction all the time. But getting saturated and/or standing water in enclosed places is a problem. If the tile was installed on backer board - as it should have been-- it will survive some wetting of the subfloor. If the subfloor is really wet or there is no tile backer board, you will probably need to replace it. Again, I think the answer is to open it up and see what you have. You may be lucky and just need to open it and place a fan in there to dry it out.

You need at least 3/4 inch subfloor, thicker if your joists are further apart than 16 inches. See if you can lay down another sheet of subfloor on top of the existing, otherwise, you'll need to replace it. Thicker subfloor will not deflect as much, better supporting the hardwoods, and giving a much more solid feel.

Honestly i think thats a decent price, for the amount of work they are going to be doing, i dont think thats outrageous. Tile tear out can add up regardless of how much is there, depending if its a mud base or just cement board, it takes time. Are they adding quarter round or just taking it off and re-installing it? either way thats still more time, if they are adding it, is it white? or are they having to stain it? thats all time and money there too. For me the toilet install i would be calling a plumber to do that, not that its difficult but liability speaking, if i were to mess it up and the bathroom floods, thats on me, hiring that out cost more but takes the stress off of me if something goes wrong. then are they adding 3/4 plywood? the plywood it self is no expensive but you have a small house, and with that means lots of cuts and that there is time consuming. Honestly speaking here, the scope of the job doesnt seem too difficult its just really going to eat some time and i think for the quote you got, it feels right, im not in your geographical area so that could be way high, but in my area, i would be charging a lot more for that same project. Getting more quotes is a good idea, but rest assure that your quote you have now sounds like a good one.

The tile is the stuff that needs the THICKER subfloor. Is the tile in good shape? Did you see cracking of grout lines or popping tiles? These are all signs a subfloor is moving too much for the tile to remain properly bonded to the substrate. Wood is more forgiving - especially if you are running the wood PERPENDICULAR to the joists. So long as the wood is running perp. to the joists you *should be fine. A thicker subfloor is nice, but may not be necessary. You have the joy of having plywood (not OSB) as you substrate. It is probably doing all the work needed. What is the flooring on the rest of the level? Is it carpet? When it comes to OSB, you can have great OSB and you can have sh!tty OSB. For that reason I like the safe option of using plywood (although some call this overkill, I like plywood's ability to grab nails/cleats/staples etc). It is also very easy to GLUE to. Some OSB products are TOUGH to get glue to grab on to. And a wide plank oak (anything 4" or wider) will require glue assist (glued down to subfloor) as well as mechanical fixtures (nails/staples/cleats). Once you are down to the 1/2" plywood, are there any floor height-changes that you need to be aware of? How about door clearance (internal and external doors)? So long as you have all the clearance you need, the plywood is a nice option throughout the home. The cost of plywood for the whole home may be part of the whole decision. A nice plywood runs about $1/sf. Installation is another $1 - $2/sf for labour. With glue-assisted installs (or any permanent floor in general) it is nice to have a layer of subfloor. That way if future removal of the glued down product is needed, it is easier to detach the top layer of plywood from the bottom layer of plywood. It means the demolition does NOT get down to the joists (that's tough to work around....12" wide HOLES between the joists with nothing but 'air' between you and the ground below).