This would in some cases force triple E.Reprise, which costs 30% as much as a proper combo to start, 11% once you account for the Verfinishing rebate. To balance this, the entire 6 step sequence beginning with triple Reprise would probably need to no more than baseline Dualcasting damage. Dualcasting Verfastspell-Verslowspell is 144 p/s. So a triple E.Reprise option that balances for 15s of completely free mobility would mean E.Reprise would have to be about 40 potency per cast.
Fifteen seconds is a long time to be completely mobile for just 7|-4 Mana (the rebate from Verfinishing being 8|19). A proper combo costs 42|31 and only grants 12.7s of mobility, the first 3.5s of which force melee range. I would prefer the solution of E.Reprise costing a Polyglottish stack, which would add just 2 free move GCDs per minute.
Procs are procs. Randomness and occasional overwrites are their nature. The procs in general aren't worth enough to worry about: the gain of 20p and 1 mana that you overwrite 0 to 2 times per 2min cycle still adds up to a single lost Jolt cast over the course of most 12 minute fights.