Microsoft was convicted of monopolizing the market for IBM-compatible PCs, i.e. not Macs.
Which makes a lot of sense, because you couldn't run Windows on a Mac nor MacOS on PCs from the likes of Dell or IBM, and you couldn't run third party software for Macs on Windows or vice versa. By contrast, you could run various types of Unix on a Dell, and run Windows software on OS/2 or DOS software on DOS competitors other than MS-DOS.
That distinction seems like it might be relevant to the current situation.
This is utterly irrelevant. I don't know what point you're trying to make.
It remains objectively inarguable that Apple does not have a platform monopoly on (ARM-compatible) smartphones the way Microsoft did on ("Intel-compatible") PCs.
Are Apple's phones compatible with other ARM smartphones? Can you install Android or LineageOS on one, or install Android apps on iOS, or get iOS apps through Google Play or the Epic Games store?
No. Also irrelevant.
It seems extremely relevant to the market definition that the alleged alternatives aren't actually substitutes for one another.
If you have a car that runs on diesel fuel and there is only one company that sells diesel fuel, it seems like you want to claim that it's irrelevant and isn't a monopoly because there is another company of the same size that sells gasoline. Is it not relevant that you can't actually use that in your car?