In the process of buying an S that supposedly had the IMS bearing done, need advice..

[78F350]

Do you guys have any advice or way for me to know definitively whether the IMS bearing was done?

The shop said it was done ant that they use LN bearings, so... what is your real question?
You have to remove the transmission from the car to see the IMS bearing. Otherwise it is a matter of paperwork and trust.

On the 986 I just got, the seller wasn't sure what had been done with the IMS, but thought it had been done. I dug through the paperwork and found a shop work order that listed "Update IMS Bearing" and had a number: 10-0124-100
There was no other info other than the parts cost and labor. I searched "10-0124-100" and found that it was the part number of the original LN dual row ceramic bearing kit. I found out that bearing has a 6 year 75,000 mile service life.

As a 2002 model you originally had a single row bearing, so a different bearing kit would have been used. Check out this page and see if your car was registered (last six digits of your VIN in the search box)
Does my car have it already? – IMS Retrofit (My '99 that was done by a shop in Tulsa was registered. This recently purchased car was not. I still trust that the bearing was done as stated.)

If the bearing was done once, will it have to be done again later?

The IMS Solution does not need to ever be replaced. All of the ball and roller bearing IMS retrofits do have a service life. Mostly 6 years or 75k miles.

Check out the various links at LN's pages. There's lots of factual info there. https://imsretrofit.com/ims-101/
https://lnengineering.com/products/ims-bearing-kits-for-porsche-m96-97-engines.html

Blackstone said that the metal shavings weren't that bad, and that they would check back after 3,600 miles

Check back with a new sample in 3,600 miles. It may be IMS related or not. Could be less metal than a 15 year old Toyota generally sheds in an oil change interval... or not. Don't Panic.