Our current system has each state’s Secretary of State heavily involved in the election process, and I believe, certifying the result. However, it is well known that these results can be challenged in court, and often for a long time.
So, on Election Day, even if the various unofficial tallies show a winner, state irregularities could yield uncertified vote totals in a significant number of states... a half dozen to a dozen, I’d say.
So, what happens... the electors cast their votes for the candidate who is probably president, but while the vote total is still unofficial?
The opportunities for third-world shenanigans boggle the mind.
Fortunately, I too believe that the ban on interstate compacts should hold and make this proposal unconstitutional. But, the legal system has degenerated in the sense of becoming politicized that you can never be certain of any outcome.
Two points: First: Yes, writing this as a 'compact' makes enforcement of it unconstitutional unless Congress has agreed to it. As "democracy" sweeps the land, it is by no means uncertain that Congress will refuse.
Second: Without the 'compact' feature, State Legislatures are perfectly free to assign their EVs however they choose.