I don't see why defeating the league with the first form of your starter would be an issue. There's probably at least one area where you can grind for levels without running the risk of encountering any new Pokemon that you either have or have yet to see due to required battles. And if you aren't strong enough to defeat any trainers, you can just keep going back and fighting them and, as long as you get some experience, you'll eventually level up, giving you a higher chance to defeat them, which achieves the effects of grinding while also spending that time to defeat the trainer that you're stuck on. This won't work if you can't even defeat one of their Pokemon, but as long as you can, it's possible. That's how I ended up doing the Elite Four in Emerald, because I was tired of grinding in Victory Road, and I ended up actually defeating them before I expected to. Also, you aren't restricted to just one Pokemon... I know the question says, "This means that you didn't encounter or catch any wild Pokémon," but would there be any issue with catching a Pokemon you've already seen in your Pokedex? Obviously you'd be very limited and it'd be entirely up to luck that you encounter the correct Pokemon, not encountering any wild Pokemon at all would be even harder to pull off than that, and completely luck based.
Also, keep in mind that this entire situation seems to be hypothetical, as even just not encountering any wild Pokemon you don't already have in your Pokedex would be near impossible. The amount of time this takes to do shouldn't matter, so the amount of time it takes to grind to level 100 with low level Pokemon shouldn't matter, as this situation obviously isn't realistic.