That's different though. Problems as a result of choosing to use a boxer engine are something that someone should recognize and research prior to purchase. The fuel filter thing is just an idiotic decision on the OEM's part. There's nothing stopping them from putting it in a more accessible location. Heck, I might get some AN fittings the first time I have to do mine and do it for them.

You cannot remove the in-tank filter & make do with an external as easily as you think. If it's anything like those found on the Camry & Celica which I've done they're a canister filter housing which holds all the in-tank components in place, ie: fuel pump, fuel pickup strainer, sender unit/float plus wiring. It's an easy job to do just takes more time.

As for the plugs - well I've heard with the right tools & patience it can be a DIY, obviously smaller hands help.

Would it void warranty if this work was performed by another mechanic that wasn't Toyota? Eg: Subaru specialist?

I can't see it being an issue, just keep proof of plugs & that it was done, ie: Tax Invoice from mechanic. I will attempt to do this myself when the time comes as I do with all servicing.

I was warned of this service by the manager of the Toyota dealer I purchased from. Said he's only heard of one 86 having it since they were released.

If it's done at 60 months it's after the warranty period anyway, 3 yr warranty here in Oz, and there wouldn't be too many owners doing 96,000kms in the warranty period. 32,000k's a year for three years would be a mighty effort.
I'm currently at 23,000k's after 4 years, less than 6,000k per year so if my usage stays the same I'm likely to be at just 35,000k's when this major service is due.

There's a how to on the forum re the plug change and mentions the special tools you need.