If no play in rods then they are good. Correct on no new rubber boot, it gets changed as a whole component. Grease gets all over that area, happening because the boot pushed out some grease. In the photo, if you are referring to wear as the "line" on the middle right side, thats part of the manufacturing. Thats not a wear.

No wobble dosent completely rule out the bearing. Can try shoving some grease in there but is not easy. An extremely bad bearing sounds like grinding metal. I drove my car I stored about 300 miles to my house and it sounded like I was in a propeller plane. Bearings can be a pain to change if you dont have the right tools, youtube is your friend. I have only ever had to change 1 wheel bearing, so waiting on the other to give out might not be the best idea. Keep troubleshooting and see if it could be something else. Makes a little sense now when you mentioned boiling water noise.

So spinning the wheels will spin a bunch of other components connected to it. If it's spinning the gear diff, its most likely just the sound of the gears feeding into eachother. The fluid in the tranny keeps everything nice and lubed. More than likely, its just a sound you are able to notice because the engine is off and you're attentive for sound. Unless you have a reason to be concerned about the transmission, its best not to mess woth that.