Well my final and last PF jars which I made half a month ago seemed to have stalled their colonization and I am 100% sure that there is insuficient gas exchange and too much CO2 built up. So my question is how can I restore the gas balance in the jar ... should I perhaps use a fan to blow directly at the jar??

Well next time you want to drill some more holes in your containers for your cakes..
for now, you might want to risk opening them up, done inside a desinfected flowhood with clean gloves it shouldn't pose too big a threat for nearly colonised cakes. If you don't feel comfy risking all of them maybe try this with half of them, and compare after a few days.

So why do you think that the PF tek works for most people without any need for extra air exchange ?

I've seen cakes fully colonise in sealed jars with an air gap on top, so they really don't need much air exchange.

I'd suggest it's an issue with moisture levels in the rice/verm mix, you can't blindly follow a recipe for the PF tek as different grades of verm will hold different amounts of water. You really have to do it by feel, if you can any squeeze water out of the mix it is too wet and what you described is the normal result.

I wouldn't recommend opening a partially colonised cake, if you want some more air exchange just turn the cakes/jars upside down after about day 8-10 after inoculation. C02 is denser than air so it will fall out when the jar is upside down.

In my experience, PF jars can look uncolonized in a corner, but actually be 99%. I think the bottom corners sometimes do get gas-locked or dry out and there can be a little patch on the outside, against the glass, that just takes forever to finish. I decided not to do cakes and wanted to spawn the pf jars to a monotub instead. I waited as long as I could then spawned them all at the same time with some still showing those uncolonized patches. But when I slid them out of the jars, I realized it was just the outside grains which I brushed off a bit before simply crumbling the cake into the bulk substrate.

I wouldn't try to add more GE at this stage. Some jars take longer than 2 weeks, give it another week and see how it progresses. By then it should be pretty much done, I wouldn't worry about the corner patches like I said, and you could probably birth it with a casing or spawn it. Or if it still has a big uncolonized area then that's a little more suspicious, could be stalling to a hidden contamination.

Ok , yeah it could be a wet spot aswell I recently read an interesting thread somewhere about puting dry coarse verm on the bottom of the jars so that you don't get the wet spot with the risk of stall or contaminants ... but anyways I'll leave it to them I left them open for 24 hours and now it seems they have secreted some metabolites I guess so they can combat the airborne contaminations

I would spawn but I do not have a monotub can I do it with a smaller tray and perhaps put it in my Fruiting Chamber?

Yep, a tray in the sgfc should work fine. But a monotub is really easy to make too, could look it up and make one while waiting for your jars to finish; that's what I did.

After the contamination issues I've been combating lately, you sound absolutely crazy for opening your jars for 24 hours. PF jars are a little more resilient than others with the dry verm top layer, but not immune. I hope they don't all contaminate on you.