Yes, you're being paranoid.
If it was installed correctly, there really isn't much of anything that will go wrong with it from sitting - most septic problems are from using the system and not maintaining it, leading to material that should have been pumped out getting into the drain field and clogging it. If it's not used, that won't happen. The materials do not degrade in the ground. (Orangburg pipe does degrade, but a system built circa 2004 is a good 30+ years removed from the end of Orangeburg - nasty, hateful stuff - basically tar-paper pipes.)
If you keep hunting you can probably find someone who will take your money to run an inspection camera into the pipes. If you like, you could dig up (or open the access manhole, if your area was moving into modern ideas 10 years ago and did not allow the access to be buried below-ground) the septic tank access - but DO NOT stick your head into even an unused septic tank - they can still collect/generate poisonous gases and should aways be treated as if they were, in fact going to kill you if you did that - use a flashlight & mirror on a stick, or hire a pro. If the system is accurately mapped, the distribution box could also be dug down to for a look. If it passed inspection 10 years ago, there's not likely to be much of anything to see in either place, though the tank at least should be looked into (Title V inspection may, in fact, be required before sale, even though it's unused as yet) just to make sure it hasn't shifted or something, and to be sure you know where the access is if it's not above-ground access (consider adding a riser and manhole cover if you have to dig to find the access.)
Don't be surprised if you find it full of water - rainwater seeps in and won't flow out until the liquid inside reaches the outlet level. 10 years allows for quite a bit of rain to fall.
If it was not installed with a particulate filter on the outlet, you might want to have one retrofitted before starting to use it. It may have been - 10 years ago those were starting to become standard in many locales.
You can install wood blocking between metal studs, but you'll have to open up more of the wall to attach them (you obviously can't toe nail them). Notch the block so it fits into the channels in the studs, then just attach it with screws going through the studs.
I wouldn't hang a sink on tile without some sort of additional support. Even if the tile were strong enough to hold it, it would provide a point of leverage against the wall that could introduce flexing.
Best Answer
The information I have seen is that the ADA recommendations for grab bars in bath and shower are placement 33" to 36" above the floor of the shower or tub. So the expected height of the shower floor or tub must be added when placing blocking during framing.
Be sure the blocking is in the plane of the studs so it does not interfere with drywall placement. And if the blocking would be placed behind the plane of the drywall so that there was an air gap between the drywall and the blocking then subsequent installation of grab bars on a tiled finished wall could bend or crack tiles.
(I had a case like that and where I was careful to gingerly tighten the screws holding the grab bars when installing them in a tiled shower. There was an air gap of several millimeters between the backside of the cement board and the blocking. This came about because the bathroom had been renovated down to the studs and although blocking was installed in the plane of the studs, the cement board was less than 1/2" in thickness and shimming was subsequently placed on the studs but not on the blocking.)
It might be wise to use kiln dried lumber K-D 2x8" for tub/shower blocking to avoid problems from warping.
EDIT Blocking for towel bars might be considered overkill, but in our house the original towel bars loosened and pulled out. What an irritation! I then used a certain type of 24" grab bar for towel bars and screwed them into studs (ours 24" OC). This limits placement.