Multi mode Powerplay is about maths. If you read Sandro Sammarcos proposal he and FD admit its far to easy to defend, leading to a static gameboard. Its why uncapping UM was suggested, because it then puts the pressure back onto the defender. In a multi mode context this would be a grind without end, while in Open it would focus players on that spot so you could drive off Uming rivals as well as fortify, providing more options to defend as well as attack.
And as I've explained twice now, you can't do this. Any attack has to fully UM a rivals most profitable systems and for them not to fortify them- a situation thats incredibly rare because each power sees them as a priority.
Take three examples: Mahon has been attacked twice top to bottom, and was completely fortified in five days. Utopia was attacked in a similar way and was fortified everywhere in three days. Now, if you can level an entire Power and UM everything and that still is not enough, what is? To get to this state in the first place requires months of patience, and even then its trivial to escape- that is unless 5C take things further.
Open gives you options, as well as making your opposition (and you) unpredictable. Taken as a whole (Open to unify playerbase, uncapped UM hotspots and unified fort direction) it does an awful lot to make things much more tricky defending- which is good, because it makes large powers more difficult to hold, and in turn allows more decent systems to be broken off to be fought over.
Well then, that seems like a problem of the fundamental game design. Because I'm not convinced that open only would have any significant effect on this problem, as you've laid it out. Defenders would have home field advantage in every circumstance, so assuming equal effort on both sides, any defending power will still end up fully fortified, just like now.
The outcome would remain exactly the same, the only difference would be the haulers would now be wholly reliant on the combat pilots, making them second-class citizens. Necessary ones perhaps, but still second class.
But in the end, open-only wouldn't change the fundamental problem at all.