Any method to assure long-life for the various plastic parts?

I think it's mostly road-grit jamming the mechanism that breaks the push-in plastic fasteners. Our 3rd gen has a plastic underpanel with about a dozen such fasteners, under the engine bay. I take the full panel off at each oil change, and before reinstalling the fasteners I rinse them out with hot/soapy water, work the mechanism back-and-forth. Take extra care when coaxing them out too, I find this style of paint can opener helpful:

 

If you have plastic panels fastened with M6 washer KEPS bolts, the washer is not wide enough and with dirt and other road debris, will "saw" into the plastic and cause the fastening hole area to degrade.

Use these washers to extend the life of the plastic panel's fastening area.

1/4” x 1” OD Stainless EPDM Washers, (50 pc) Neoprene Backed, Choose Size & Qty, by Bolt Dropper

 

Plastics in areas with sun exposure do benefit from an occasional hit with your favorite UV-blocking protectant. (It's about time for me to go do that again. I like your radio idea, maybe not football, but I should find something.)

For things like the fasteners down under the car, that's a bit less of an issue.

 

For external plastic protection, I use 303 Aerospace Protectant. I use it also to dress and protect the tire rubber once a year before storage after winter/summer tire change. I have not done a side-by-side comparison, so I don't know how effective they are in real-life, but it shines and the luster looks good. I once used it on the all-weather floor mat. It looked fantastic but made the floor too slippery, so be careful where you apply this stuff.

Another thing I have been using on rubber gasket around window and hutch door trims is Gummi Pflege. I started using it when we had a minivan with a sliding side door that got frozen shut during cold winter. A thin coating of the Gummi Pflege application solved this problem, and the rubber stayed soft and supple even in sub-zero temperatures.

As for the plastic fasteners on body parts, I have given up on the idea of reusing them. I just buy bulk aftermarket fasteners of the same diameter and length. And any fasteners that need to be removed in order for the car to be serviced, I replace them every time I take them off. It is easier than trying to remove it intact and reusing it. And far cheaper than buying OEM fasteners.

 

The one car I ever bought new was a Mazda 323, and one amazing thing about it was the steering wheel. I don't know what they made that thing out of, but it felt perfect. It wasn't soft exactly, but had a certain slight amount of give in the surface, and just the right amount of very slight tack, so gripping it somehow felt the exact way that gripping a steering wheel ought to feel.

In my first week of ownership, I was behind a truck that a rock fell from. So I was back to the dealer's body shop.

When I picked it up, they had not only fixed the rock damage, but given me a 'complimentary' spray of Armor-All on the interior.

Steering wheel never ever again felt right.

 

Yeah I quit using Armor-All (and it's ilk) at least a couple of decades back, and unloading cans of Scotch-Guard on the upholstery.

 

Yeah, for a similar reason I do not use "protectant" inside of our car. I don't remember what brand it was, but I used one of those vinyl/plastic protectants on the dash panel and other hard surfaces in my car once. It looked gorgeous, lush, and shiny right after application. But, the smell was unbearable, and if it was applied to the area that needed to be touched, then the surface remained sticky even after the drying period. The worst thing was that it looked good for only a very short time, soon after, it just collected more dust and dirt. And made cleaning a lot more difficult. It could be an Armor-All, but maybe another brand. I thought most protectants will be similar, thus never used again inside of the cabin.

 

Another vote for 303 Aero here. Works well against UV without the armor-all problems.

Cleaning the fasteners before operating them helps as @Mendel Leisk detailed above.

Another: don't try to pop them on super cold days. It really is worth letting the car warm up in a garage for a few hours first.

 

I've tried lots of different black trim restorer products that get rid of oxidation... Most of them don't last very long, but a car detailer on TikTok uses a version of this product with black dye in it and claims it lasts a year or longer. Still haven't tracked that one down, but look forward to trying it.

 

I'm not sure there really is much you can do to extend the life of plastic fasteners other than keeping the car garaged as much as possible, and out of the sun. When new these small plastic parts contain plasticizers which tend to keep them flexible. As they age those evaporate while at the same time ozone from the air attacks the bonds in the plastic, and mechanical and thermal stress cause little cracks in the material. (Ozone is a big problem in SoCal, maybe not so much in Montana.) After 15 years everything has become very brittle and the only good solution is to replace those fasteners. For instance, a few months ago I cleaned the fan for the big battery, and several of the fasteners that hold the deck down onto broke, even though AFAIK none of them had ever been out in plain view before (our 2007 still has the original battery).

This site has a very wide selection of these sorts of parts, all for a fraction of the Dealer's part department prices:

https://www.clipsandfasteners.com/

 

I've replaced a lot of those undercover nylon 6-6 plastic fasteners with rivet nuts and stainless steel screws as well as large stainless steel neoprene rubber backed stainless steel fender washers. The neoprene backed stainless steel washers as designed to survive from constant UV exposure on roofs and metal commercial buildings.