I think you should try it, Dave. I also think - for the future - that much more player control over supply would be great (I would like to be able to manually plot the supply routes, for example).
Will your fix solve the issues that have been raised? I think two broad issues were raised.
1. Continual attrition of supply resources through interdiction due to the AI repeatedly making runs into interdicted territory.
2. Territory interdicted when it shouldn't be. In particular, examples are in the threads of, for example, almost surrounded units managing to interdict the supply to the units surrounding them despite being under suppressive fires. But that's just an extreme case. Take the Oosterbeek pocket - what I would like to see is that a pocket such as that would work in the game as it would work historically. Historically the FLOT in the pocket got barely any supply because none was dropped into the pocket. The little supply that was dropped directly into the pocket, and the supply they did have within the pocket already was successfully distributed by hook or by crook - we assume it was man-ported. But in the game, if you do drop supply into a pocket, for example (place a SEP there, get your bases in etc), or a bridgehead, then the sheer weight of enemy beyond the FLOT can regularly interdict ALL supply to affected FLOT troops. When this happens in a built-up urban environment rich in cover and concealment to help work supply through, then the modelling seems very off. Stalingrad couldn't happen, for sure.
It sounds like your fix would go some way to stopping repeat 100% interdiction messages - solve the first issue. The pay-off would be - what? That the player would have to change the combat/supply base situation to get supply through, because if everything remained as it was the AI would continually determine that the route was blocked. That sounds good, but it crosses into issue 2 then, especially if the situation is in terrain rich in cover and concealment possibilities. Because often the reason the AI would determine supply as interdicted would be due to the circumstances arising in issue 2.
The fix wouldn't address the second issue, I think? Is it realistic for enemy units to be able to interdict supply to the FLOT not by cutting the supply line in the traditional way (attacking a choke point on the supply route etc), but simply by being engaged with the units needing supply, so that the mere presence of the enemy units in effect cuts the supply route to the unit right behind it, at the very last part of the supply route. That's what seems to happen at the moment. It makes it impossible to secure a supply route to engaged units. And some examples suggest that the enemy interdicting supply like this needn't have a force superiority to do so. Though, it doesn't happen all the time, all the examples posted of airborne ops show this, I think. The 508 at Nijmegan get cut off by the amount of enemy armour coming over the bridge, or at the other side of it, regardless of whether there's a nice safe avoidance route into their rear. When they're in an urban environment, as they are, this seems wrong. It seems less wrong when they're in the open, but it can still happen in a counter-intuitive way when they're in the open - for example when Allied units are interdicted by the presence of an Axis unit that they outnumber and outgun.
For the second issue I think you would need to have a look at how the interdiction calculation is made, especially how those units right in front of an interdicted unit contribute to the calculation.
Great you're back and looking at it, anyway.