I presume you mean by “car on” that engine was idling. If so, those voltage measurements are good, they do not indicate any problem with alternator. Auto batteries contain sulfuric acid so could conceivably produce a sulfur odor, but in 40+ years of diy’er car maintenance & repair, I’ve never noticed that. I’m skeptical of the battery/alternator theory as the cause of the sulfur odor myself. Ask shop to thoroughly clean the outside surface of the battery and the connections. If the battery cables are very corroded, ask shop if they should be replaced.
Ask shop to assess braking function. The sound is probably just the pedal bushing squeaking, or the brake master cylinder rubber boot rubbing.
re: cat
Cat function is tested by a standard test on OBD II cars like yours, w/shop mechanic comparing time waveforms from pre- vs post-cat o2 sensors. That’s the only way to assess the cat function. Uusally done w/a pro-level scan tool. Your car’s drivetrain computer does that test automatically as you drive, and if there was a problem it would usually produce a diagnostic code (P0420 probably). I’m skeptical of any problem with the cat function. If there’s a sulfur odor from the cat, that would be caused by the gasoline used, not the cat. By switching to a different gasoline brand it seems like you’ve already solved that issue. Odors from the exhaust system more likely caused by exhaust leaks or oils and fluids (from engine & transmission leaks) dripping on hot exhaust components.
Unlikely to be related to alternator or cat. My guess, your water pump seal has a very small leak, which evaporates from the engine heat so isn’t visibly noticed expect by a slight loss of coolant. Suggest to monitor the coolant level and top off as necessary. If more than one cup of coolant loss per 1000 miles, report back. Next time at shop ask them to inspect water pump for signs of leaks. Often can spot white residue in leak area, even if no coolant drips seen. I had a leak of this sort on my Corolla, but could never see where it was leaking from until I eventually cleared stuff out of the way enough to replace the water pump.
Me, I wouldn’t do anything until an actual measurable symptom appeared. If you plan to continue driving this car, keep it to 150k, 200 k miles, no harm done to replace those parts pro-actively, as long as the replacements are to oem spec. Note that most parts-store replacement parts are not oem quality; oem versions often only available at dealership parts department. Replacing a working oem alternator with a non-oem version, not something I’d do. Likely to end up with a worse alternator that you started with.
If you are looking for something to replace proactively on a 100K miles widely sold sedan like the Sentra, these would be my first priorities, depending on what an inspection showed about their condition:
- tires
- brake pads/calipers/master cylinder
- radiator & water pump replacement
- exhaust leak test
- timing belt (probably not applicable to your car)
- fix engine oil and transmission fluid leaks. Most common engine oil leak is valve cover gaskets, relatively inexpensive job