As far as your bullets above the conclusion go, I respectfully ask why you are playing in open then, if there are no benefits to it?
And what exactly is the unfair advantage that you believe people switching between modes is? While the argument that solo players progress in the game faster keeps being given, where exactly is the proof? Yes, they have no risk of interdiction by CMDRs, but can certainly get interdicted by NPCs, and in any case, surely a persons progression in trading is going to depend on all sorts of variables, such as the size of their ship, the lucrativeness (if that's a word) of their trade runs, not just whether they get interdicted by a human CMDR who takes 5 tonnes of cargo every couple of runs. And don't forget, players trading in open and NPC's also affect the market. I really think this argument is just poor cover for PvP pirates wanting more prey. And what progression are you referring to? Accumulation of wealth / ships? Plenty of people who trade in open say they do just fine, so presumably you are not at a disadvantage there. Progression in combat skills? Plenty of people think that NPC's aren't great fighters, so their combat skills aren't going to improve faster than yours in open fighting real CMDRs. Reaching Elite status? So what? My understanding of ranking progressions is that only progressions made in open play get counted, at least towards the 'Race to Elite' so that shouldn't be a worry. Just because a CMDR has a higher combat ranking than you doesn't mean that you will lose to him in a dogfight.
Everyone has the choice to play open, solo or group, including the people who are complaining. I believe the general consensus on the forum is that most of the vocal pro-PvP lobby are players who wish to play as pirates and want more prey. I could be wrong, so I'm not stating it as a fact. The other complaint that get's thrown in and equated with cheating is that pilots in solo will acquire huge wealth, a lovely Anaconda armed to the teeth and then come into open play just to mess with the PvPers, which is so, so unfair.

Especially as some of those PvPers will have spent the time messing with less experienced and lightly armed traders, but I guess that's Ok.
You obviously feel strongly that you are being disadvantaged by choosing to play in open, and that's a shame. I really don't think you are, as this game isn't a race to get somewhere first, certainly not a competition, and I imagine that the little competition that Frontier set up as the race to Elite is not the focus of that many people playing the game.