Custom games were meant as a player VS player environment.
Practice Tool isn't meant for that purpose. By no means am i saying that this is the extent of the Practice Tool we are ever going to get, it's pretty much the most basic thing Riot could've done when they still have a lot of work to do with the new client. I'm sure they'll add more stuff to it, just not the ability for other players to connect to your practice game.
There are probably solutions that involve simple waypointing bots that use abilities, giving them simple commands that can solve a lot of practice issues, coming from the lack of interactions from the champions themselves. These solutions are a set of unique challenges Riot will have to figure out how to implement, if they ever decide to in the future.
Riot won't remove me and won't be able to link my accounts to each other.
I think you're underestimating what Riot knows about you. You're playing their game and they know from which computers you play. Each computer has it's own identification that's gotten from the unique IDs of its components. Essentially, if they ban the access of your components, you have to physically replace them with new ones to play the game again.
You can hide behind VPNs but your computer's ID is unique and doesn't change, like your own fingerprints that uniquely identify you as a person.
It doesn't matter if your VPN has the IP address of a specific planet in the Andromeda Galaxy, it still says that your ID is playing the game from a different location.
This isn't meant to scare you in any way. It's just a confirmation that Riot isn't dumb.
If Blizzard can go as far to crack down on trolls and cheaters to such a degree, what makes you think Riot isn't capable of it? But for the moment, Riot isn't going that far to hunt down people who wrong their playerbase. Their lax attitude isn't something anyone should take for granted.