How does one obtain a Witcher 3 Troll Mutagen from a non-quest specific troll, at least one of which you never exchange dialogue with

Triss can help you with the statuette quests. It depends when you find them, but before the battle of Kaer Morhen you can talk with Triss and she will help.

To update - there is a time after battle of Kaer Morhen, when she is freely available to just talk and not in the middle of any quest. Also, you can talk with her after the main quest line ending, then she will help to decompress the statues.

There is no way to scale quests or their rewards to your level.

It wouldn't really make sense narrative-wise if delivering a pan got you 2000 gold and a rare Witcher sword, would it?

Since side quests rarely affect the main story (typically ones involving major characters from the books), just do side quests in order of ascending recommended level. If you're already way past some quests' recommended levels, just do them in whatever order you feel like. The easier quests typically won't take nearly as long as the later quests, anyways.