As far as I know the main difference is that in japan the DF is used to clear content, after you've learned it before in PF - NA (and EU) are having it the other way around. The western world just queues up for that content as soon as its unlocked, japan practice it first. So maybe a roulette like that would work in japan but not in NA/EU because of the different mindsets. And I wouldnt say one of them is better than the other one - both are valid options.
SE has given us more and more reasons to re-visit those fights: music, Khloe, carfting recipes that require drops from there - on top of the actual drops and the mounts. And at least I dont want to revisit the same old fights over and over and over again - even for the sake of new people.
I also cant see what sort of reward would be good enough for people to do that again - I cant imagine a simple tome-bonus being enough.
(Either way I really believe the way to tackle this today is the same as it was in the old days: Form a static, maybe for recent raid-content and ask them if they'd be interested in a fun night of learning old ex-primals - we did some coil-synched runs for fun once... tried T5 with minimum ilvl and failed - first because of, well failing mechanics, every try after that because we were laughing to much)
That depends on the fight and the people who are new - how much do they know about the fight? Which roles are new? How much preasure is the raid putting on those roles? Even a complete new tank could MT Titan EX, a new healer might be more in trouble. Some fights are simply easier to carry than others, so I wouldnt fully agree with your friends statment.
I also want to remind you again of the variety of difficulty those raids and trials have - I have cleared T1 countless times in DF when I was farming ARR-light. I have tried T13 once or twice around the same time - and we didnt even made it past the first phase.
And again: in japan the mentality is not to enter the DF unprepared - thats not shared in the western world.
This actually correspond well with my point of huge diffculty variety between the raids and trials!
I also dont support your idea that the problem is having these in the mentor roulette - yes, you might hope for a guildhest when queueing up for that, but you know you can end up with an ex-primal. The problem here isnt the mentors, its the people who know nothing about the fight - and that 60 minutes arent enough to teach. And that it gets REALLY frustrating to see people making the same mistake over and over again even though you explained it.
I've done Garuda EX through DF a LOT and people kept killing spiny - despite marking it with "ignore" and telling them over and over again not to.
I've failed Ultima HM because the other tank didnt had provoke unlocked.
I've seen people in Shiva not understanding what "Stack behind her" mean.
I've done GKMM EX in DF and struggled to convince a bunch of germans after two wipes that no, this isnt like HM and that you really, really dont want to kill any of the moggles until all of them are low in health and that there isnt a kill-order like in HM.
I've tried Ramuh EX and I still have the macros I used to let people know when to collect orbs - and even with those I may or may not have destroyed a keyboard over that fight...
I've already mentioned how GREAT T13 was...
I'm not enterily against a roulette like that - but I doubt that it will really help. And looking at what your friend said - about how you need 4 experienced people and the rest can be new - it sounds a lot like a carry. How is that different from doing this content syched? Oh cool, you've seen the whole fight now and it took 10 minutes instead of 3, grats! You still have no idea how mechanics work because the first of Titans landslides send you flying, Levithan knocked you of the boat as soon as he could and your party in T9 asked you to run into a wall so you wont get into the way after you caused the first wipe.
I understand that you want to learn those fights and see them the proper way, but again: I dont think DF was, is or ever will be the real solution for that - wether you have a roulette that makes people bail on Ramuh because its well, Ramuh, and not Shiva or not.
Want to learn this fights? Form a party of your own! Wether its with friends or through PF!
How do you think all the experienced people learned how to do them?
The old EX-primals are, well, old - if you want a challenge, there are new ones. New raids. So doing the old ones sychned doesnt seem to harmful to me - and if you dont want to do that... I'll only repeat myself here.
The problem is not that there isnt enough incentive to queue for those, the problem is that at least in the western world DF isnt the way to go for EX-primals