It's not calculated until the end of the cycle. Note "lasts for a single cycle change".
A control system can be fortified on Friday and undermined on Saturday, in which case at the end of the cycle it is neutral.
Yes, it's neutral. The Fortify cancelled out the Undermine. Without the Fortify you'd be paying more upkeep. So it was partly successful.
So you are worrying about players from other powers undermining in Solo? What about players from your Power Fortifying in Solo? Or players from your Power undermining another in Solo. It all balances out.
You want to stop everyone else from Undermining your system completely? Tough. The game wasn't designed like that. It was NOT designed for players to oppose other players via combat. They do it through
indirect actions. You seem to have misunderstood the focus of the game. Actions have counter actions. You oppose actions with their counter action, not by blocking the action in the first place.
The modes do not open a loophole or a flaw, they expose the game as something other than you are hoping for. That's all. (It works better if you think of it as designed for Solo rather than designed for Open, then it makes sense. Sorry about that.)