Can I buy SQ even if I play on PC?

No idea. The number of people who are willing to go through the hoops to get FGO to work on a PC is slim at best.

If you use soemthing like bluestacks, which is connected to the google play store, then I think it would work

FGO cant run on bluestacks, and in the emulator I used to use, Nox, I couldnt.
I dont know if there are other emulators capable of running FGO, but at least on those two, it was not possible.

I f I were you, I would transfer my account to a phone, buy the SQ, and transfer it back.

Could be quite problematic if your phone is rooted, hence it is one of main reasons people run the game on PC

Yes, you can!

I use Nox player and paid for SQ via paypal recently.
You could also use credit card. When using paypal you may have to change your google address to an American one for NA server, Japanese for JP or else there'll be an error message...

I'm not sure if you can use gift cards, but I think you might need an American google play card