But if you're uMMing an enemy system, and some one comes after you... you'll be very likely flying a combat ship, so fighting back might be part of the "plan".
The other way around, if you're uMMing and an enemy hauler drops in... you can take his/her scalp and bring it to your HQ.
Which is
why I think that Powerplay Open Only will not result in more fun PvP, but more frustration for those who
enjoy PvP.
Under the current paradigm, especially under Powerplay, it's highly likely that in the scenario you're describing, both sides will be operating under the same unofficial rules. We've opted in
twice: first to Powerplay, then to Open. They're not likely to take advantage of logging in and out to avoid PvP, let alone try to manipulate the instancing algorithm.
Based on past experiences in other mixed PvP/PvE environments, both scenarios you describe will likely result in at least
one party logging out at the merest
hint of being killed in PvP. Doubly so if PvP remains an ineffective way of
advancing your own power, rather than simply slowing down an opposing power's agenda. And instancing manipulation is simply going to make the game more unpleasant for
everyone one involved.
Don't get me wrong, I've long said that
efficiency is the enemy of fun in any game, and I have no problems with
anyone sacrificing efficiency to have fun. I do it all the time... though oddly, a lot of what I consider fun is also more effective compared to common grinding strategies.

Under the current Powerplay paradigm, I'm not likely to
ever bother with combat against a combat specced PvP ship, primarily because I don't consider that kind of grinding battle of attrition to be
fun. The fact that what I could
gain by winning the battle, vs what I could
accomplish during the same amount of time, is a secondary factor.
Whether I would do that in Powerplay 2.0 ultimately depends upon its mechanics. The best-case scenario to
me would that Powerplay becomes not a contest of merit hauling vs combat farming, but a contest of
mission running. Combat between two ships specced for mission running would
not be a grinding battle of attrition, and thus could be a lot of fun,
regardless of outcome.
Whether it'll be
effective is another matter entirely, but that wouldn't matter to
me. Under the current tri-mode paradigm, it's highly likely that the other player will feel like I do, and operate under the same unwritten rules I do. If they didn't feel the same way, why would they be in Open in the first place?
Under an Open Only paradigm, though? The players currently in Solo/PG, for
whatever reason,
won't feel the same way I do about this scenario. They're going to use a variety of unsportsmanlike tactics to avoid that scenario, and I would find that to be extremely annoying. Furthermore, any attempt to police that behavior by Frontier would be a waste of time, money, and most importantly make the game
much less fun.
The tri-mode system accomplishes something I considered to be
impossible: a mixed PvP/PvE environment where an significant majority of players
voluntariy choose to play in it, and the ugly behavior common in other games is a relatively rare occurance. And Frontier managed to do this
without a massive team of in-game moderators to police player behaviour, and oppressive game rules that are far more likely to ruin the ingame experience of the innocent than suppress the
actual problem. It's a
social solution to what is ultimately a
social problem, and it works brilliantly in my opinion.
YMMV