OKAY HERE ME OUT: I just had an idea for the Ashfur Ferncloud age situation,, So Ferncloud's brother can be Tulipkit, and they are apprenticed late due to all the Bluestar shenanigans going on, and Dustpelt doesn't mentor one of them cuz he's too young. Tulip takes Ash's appearances, and then later Dustpelt and Ferncloud get together and have their first kit: Ashkit, around the time of Firestar's Journey. (So he's closer in age to Squirrelflight.) This fixes the issue of Dustpelt not being Ashfur's mentor, because instead he'd be his father! And it fixes the problem of any Ferncloud - Ashkit - Cloudkit scenes because Tulipkit can take Ashfur's place until you either eventually kill him off or maybe he sticks around (like petty Ashfur)

I'm definitely solid on breaking the litter in half, with Fern and Elder in the older and Ash and Tulip in the younger. This sister will die during TNP, either in the destruction of the Forest or in the WindClan civil war

(or vice versa with Elder and Tulip; whatever name people like more. It's either Elderberry or Tulipflight. Also in any case they will be a sister.)

Though when Ashkit is born is still up in the air. I think your suggestion's interesting in how it would actually make Ashfur a surviving brother of Spiderleg, Birchfall, and Shrewpaw, but it seems a little too far ahead. It would remove Ashfur's grievance with Tigerstar which I think is important.

His mom was killed and he grew up without her. That doesn't have the same punch if it was just his grandma.

I'm leaning towards him being something like 2 moons old when Brindleface was killed, and Fernpaw is forced to be the closest thing to a parent he'll ever know, with Cloudtail helping out a lot. It works best-- eventually Dustpelt is close to him because that's his brother-in-law

Petty Ashfur is really popular though... maybe it would be worth it to just shuffle in some petty traits into Ferncloud? After all, the 'soft den mom' role is already filled by Daisy, maybe it would be worth it to add some more variety to camp?