Primary donate makes sense only if matched with an high quality secondary bungeed under your chin, so you are sure to get it in your moth quickly and safely after donating the primary (my suggestion is to use two identical second stages).
That is for diving with a buddy and without an additional redundant tank.
At that point, even if you have an integrated regulator/inflator, you cannot rely on it, neither for you, nor for your buddy. So you could as well use a normal inflator.
An integrated regulator-inflator makes sense only for solo diving.
In that case it will be attached to your primary first stage, allowing you to use that big air reservoir even if the primary second stage fails.
In that case you need another complete reg on the pony tank.
It is not easy to have the same setup both for buddy diving and solo diving...
The only way would be if your main tank has two valves. My 15-liters Faber steel has actually a reversed-T valve by Cressi, with two posts, two valves and reserve. With that I can mount both first stages when I do not use the pony tank...
My opinion is that primary donate always makes sense (for all the reasons most tech and experienced rec divers have gone to that, which I will not go into here). I am not a fan of the integrated octo/inflator as a rule, but I do see this as a viable option if the diver also plans to ALWAYS carry a pony. Here's why:
1) Since the diver will always have their fully redundant air source (pony with it's own 1st & 2nd stage), when the diver donates his primary, he goes right to his pony (recommended to have that 2nd on a necklace) and need not suffer trying to breathe from his inflator while taking his buddy to the surface.
2) If the diver has a problem with his primary (whether diving solo or with a buddy) he goes right to the pony and can then sort out the issue with the primary without being tethered to his buddy. It is now more possible to work on the primary and get it working again (i.e. can shut down primary tank valve and still breathe while checking / reassembling / retrying primary reg without impacting ability to breathe off the reg in mouth).
3) In any of these scenarios the octo/inflator is still there as a third tier option for breathing if, for example, the pony runs low (unlikely if exiting a NDL dive) or the pony reg does not work (also unlikely if tested before each dive).
4) This setup will work equally well solo or with a buddy, so it allows the diver to get used to diving in one consistent config that works for either.
5) This setup allows the diver to more easily transition to more robust technical setups such as sidemount or backmounted doubles with minimal changes (i.e. same regs with one in mouth and the other on necklace, except now both tanks are full sized). BTW, did you notice that 1) and 2) above align with "when in doubt, bail out" for rebreather diving. The diver will already be used to bailing out to the pony for problem solving, so this mindset can easily carry over if the diver ever advances to rebreather diving.
Note that once the diver commits to ALWAYS carrying the pony, the need for that octo / inflator becomes very minimal (only to access the full volume of gas in the primary tank if the primary 2nd stage fails AND the pony gets empty or also has a reg failure). Personally, I would say it is not needed at all at this point, as this is close enough to equivalent to independent doubles or sidemount, except that the second tank is smaller than the first.