Okay, I think I understood (a bit) your idea.
When tethering with a modern smartphone, you don't tether like the old dump phones. You do not use the phone as a "modem" to dial (you don't send "+++AT" commands to establish a 3G connection from your computer).

The phone will be connected on its own to the internet.
And then *redirects* network trafic to necessary devices.
It works like a router.
The ISP still sees your phone connected to the internet and has no clue what you're doing at all.
The laptop you're tethering too doesn't see a *modem*, it sees a network access point, like a Wifi router.

Out of the box, the smartphone has a nice GUI to enable such routing over Wifi. The smartphone simply starts to act like an Internet router, to whose Wifi you connect.


With the USB tether, once you've installed the dev mode support, the smartphone show on the USB as a USB network device.
A laptop with linux has support for that out of the box. If windows doesn't, a pretty standard USB Network driver should do the trick, nothing special needed.

This USB network works like if you had a direct network cable to your phone.
By default, it's a simply 1 to 1 network : you laptop can see your smartphone, your smartphone can see your laptop, that's it.

It's possible to type commands that will ask the smartphone to start forwarding thing from USB to its internet connection.
(Just like a router, but this time wired).

It's also possible to connect to SSH running on the phone in proxy mode and use that as a proxy on your browser.

It's just yet another app store for android.
The peculiarity is that instead of having one giant single list of software (like on Google's Playstore), Aptoid has tons of different repositories to which you can subscribe.

When you install "android support" on your Xperia, you get android apps also appearing in the Jolla Store.
One of these is "Aptoid". If you install it you can access any android app on any aptoid repository that you like.
By default, the repository called "Sailfish-app" is active and you can get tons of applications from there.

(NOTE: Currently, the version of WhatsApp there is buggy. For now fetch it from the WhatsApp website).


In that case, beware :

Google is putting more functionnality into their proprietary services, and more applications are relying on that.
(e.g.: their location system is proprietary).

A good alternative is trying to get microG installed instead. It's an independent opensource implementation of the same APIs that some apps might want, and that would require the proprietary Google services.
I haven't had opportunity to test them yet, but there are people reporting success on Xperia with these.