OK, mom'scar is about 9 years old and the belt was replaced once. The belt looks good, and there are three marks and it's about halfway to the loosest mark, so tensioner seems to be in range when the engine is running BUT

On a few cold nights, I took the car out and the BATT symbol seems to stay on longer than usual and then the MIL light but only for 10 sec max.

On one of those cold nights, I was able to wiggle the tensioner and it didn't seem to come back to the same place and it seemed a lot looser that night than a warmer morning. Today's warm was about 37F and he BATT indicator just seems to be on for a few seconds after the car starts.

Is the tensioner on the watch list? Time for another belt? Lube?
The Chevy Shop manual was little help. They seem to be about $100.

Am I barking up the wrong tree? How easy is the repair? Any tricks?

I recommend replacing the belt every 80,000 or 7 years, whichever comes first. Stay away from belt lubes. It sounds like it's also time to replace the tensioner. This link should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-5.html#post578394

Easy job. I'd go with a Dayco tensioner, the inventor of the tensioner/serpentine belt for the original Mustang. Dayco products are usually less expensive than ACDelco.

That's what I thought. The car only has about 30K miles on it and 10 yrs old. Belt was replaced once.

Thanks. Job for spring.