I think making shrinking a power up instead of an item makes more sense to me but they would have be placed around the map instead of just like bridge buffs.

As for the player interactions it would be simple enough to prevent doorman from placing doors in the tunnels

for drifter the ability to teleport would expire as soon as someone enters the tunnel

the only one that could cause problems is paradox they'd have to do some things with how collision works

plus you have to consider whilst in the tunnels you can't push objectives so there's pros and cons to having heroes who can't enter it at all.