If you were to electrify the dive-under and use it for EMU reversals, the constant reversal moves would adversely affect the freight capacity - thereby defeating the whole point of the project. Passive provision is being provided for future wiring though - but don't hold your breath as to when that might be if it ever happens.
I also fail to understand your point about "increasing the conflicts" at the South end of the ECML - the constraint at Welwyn has always been there, and King's Cross's constraining layout (which was fine in the 70's) has been set in aspic for over 40 years. There has never been a single capacity constraint, always multiple. KGX, Digswell Viaduct, Hitchin (pre-flyover), Huntingdon-Woodwalton, Werrington Jct, Allington (pre-curve) and Joan Croft Jct (pre-flyover) are just some of them from the ages - and some are still extant. It's much less "increase conflicts at the south", and more "an increase in concentration of conflicts at the south end".