The one thing I'd need from Sandbox Mode

A manipulable URF bot.

Failing that, one that stands in mid lane and casts its abilities and AAs in a predictable fashion. (It would prioritize abilities it hasn't used yet and skip the ones it can't cast without a target.) Either way, they'd reliably teach you how to play around range, cast times and hitboxes.

All that'd be left to learn after is damage output with XX items and XX runes (and XX levels) and that's something you could straight up calculate.