forester2002s

In my experience, over many years and with many cars, once a battery starts playing up, you might as well replace it sooner rather than later. You can try and be nice to the battery, but it won't buy you much time.

What was the CEL? A scanner may show it as a 'pending' CEL.

That's not how heat transfer works. The faster the fluid flow, the better the heat transfer. And the slower the fluid flow, the worse the heat transfer.

When the noise is loud, what happens if you brake gently? ..Brake harder?

This looks like the source: https://snugzusa.com/product/zlbxwt-aslpnewtsm

There's an awful smell (like fried grasshoppers?) when the grease from a split-boot hits the exhaust. But I think of it as a Subaru built-in feature, a bit like a warning light. That smell is hard to ignore, and serves as a warning to do something about it PDQ.

Found it on the street? Use it on your chapped lips? I think not. Perhaps it would work on those rubber door seals that dry out? Would smell good too.

The first of your eBay links is from Six-Star, which I think is a Subaru dealer (someone can correct me). They list all well-known manufacturers (although they mis-spelt Mitsuboshi). Total price at $240 is high but not unreasonable for OE parts. The second eBay link says "OE Spec". Who know what that means? And their total price is suspiciously low, $70 for all those components. I wouldn't touch them.

https://www.ebay.com/sch/i.html?_from=R40&_nkw=side+mirror+glass&_sacat=33649&selvel=Year%3A2014%2CMake%3ASubaru%2CModel%3ALegacy%2CTrim%3ABase%20Sedan%204-Door%2CSubmodel%3ABase%2CEngine%3A2.5L%202498CC%20152Cu.%20In.%20H4%20GAS%20DOHC%20Naturally%20Aspirated%2CEngine - Liter_Display%3A2.5L&selcontext=productType%3ACAR_AND_TRUCK%2CpartType%3APARTS%2CqueryType%3ABY_VEHICLE

Sandwiches fit into that strap nicely. My favorite: Peanut-butter and banana, on whole-wheat bread.

As a temporary measure, you could wrap the split boot with plastic-wrap (eg Saran wrap), followed by a covering of duct-tape. This will temporarily reduce loss of grease, and contamination by 'dirt'. But this will only help if the split is caught early on.

The first item Rear Wheel Bearing @ $377 seems reasonable. But the other 2 items seem grossly inflated. But I'm only a shade-tree mechanic. Others will have a better take on these items.

I've found that an oil change often gets rid of 'tappet tick'.

Could be that the clutch is failing for some reason. Perhaps it's engaging at start-up, and then starts slipping/overheating? Two options: 1. adjust the clutch (search on this forum for previous posts); or 2. get another compressor (perhaps a used one from a bone-yard).

AC light on usually means charge is OK. Does the clutch spin at startup, and then stop spinning after 3-minutes?

Maybe the compressor clutch is disengaging after 3-minutes. Can you see if the compressor is turning (not the pulley, which turns all the time, but the clutch-plate in the center)? And does the AC indicator-light stay on?

Sound like you don't have a full 12V supplying the unit(s). Try temporarily connecting a (measured) 12V to the unit, before installing it in the car.

If buying online, and there's no mention of Manufacturer's name, DON'T BUY.

Yes, fix both. But not at the same time, please. Otherwise, you (and the rest of us) will never know what caused the problem.

Foreign object stuck between brake-rotor and splash-guard?

Yes, a ground-issue is definitely a possibility. You could start by removing each bulb in turn, and seeing if the symptoms change.

My money would be on wheel-bearings.

Get it up to speed on the highway, where the vibration is very noticeable, then: - ease off throttle and decelerate - any change? - accelerate gently - any change? - accelerate hard - any change? - brake gently - any change?

Or, use a 'Turkey Baster': https://www.amazon.com/dp/B00MN62P8Y/ref=dp_cerb_1

Subaru heat shields are notorious for rattling. And your symptoms (low RPMs, light-load, decelerating) seem to point at heat shields. BUT, heat shield will also rattle with the car stationary. And the rattling can be quite loud when the engine is started. If you cannot create the rattle at zero speed, then it is most likely not the heat shields.