The floor looks like it's been worked on over the years due to the different collors in the planking.
Did you remove the roted wood before you took the picture? That's drywall on the far side of the footer so I would haveto assume that it's an inside wall. If it is an inside wall how did the moisture get to the footer to rot it out? If there's been an addition added on or something to stop future rot you may not be in that bad of shape. There's wood missing put overall the board looks to holding the wall fine, although you might want to shoot some expanding foam or something tokeep the moisture from the crawl space from getting inside the wall.
I moved to Florida from Illinois. I worked 3 summers in high school for Ma Bell (note that after the broke the Bell System up everything is owned by the Baby Bell's and GTE that changed it's name to Verizon it's gone back to everything being owned by Ma Bell and GTE again) because my dad ended up being the VP over engineering. The first to summers he was what is called the “Wire Chief” over all of Northern Illinois. Nobody wants to work with the boss's kid, but your boss's,boss's.....kid is something else. The let me leave 2 weeks early for the sheer fun of double sessions footbal practice, and that when I let them know who my dad was. All I replied to all of the you didn't tell him's, was that he'd have killed me first.
It's been a long time since I've even seen a crawl space and I don't miss them a bit. Who ever came u with the name crawl space? It should be,”lay on your back and shimy space”.
For you to get rot where the shower was just means that you had a leaky shower. The missing base boards by the shower may never have been there to allow for the plumbing.
The big question would be “how solid is the floor as a whole?
If the floor is still good and solid and after replacing any rotten boards in the floor, you could raise the drain for the shower and toilet and them overlay the entire floor with new plywood. I wouldn't trust thoseshort boards though. I wold pull those up and run 2 x 6's or 2x8's all the way to the backwall and throw a cross strut in if you still have one end unsupported. They also cut out way to much board out around the red capped pipe thatI assume is the shower drain. As long as the floor is solid your going to end up with a good solid floor.
There will be a slight rise going into the batrhroom. I always make my own floor james at doorways myself out of oak with counter sunk screws a wood dowels to give it a nice finished look.
If you slope the board there won't be a “trip over rise”. If there is carpeting outside the bathroom router that edge round so the carpet can be tucked under it. Then router the bottomsideof the board to allow for the rise. If you're going to pit tile down, remember to allow for that. But you make the jam last anyway.
That would be a lot cheaper than bringing in a pro. But if the floors not good and solid you'd have to tear up the whole floor and run your joists 24” OC. That would require brining in a pro or several friends to thelp you out.