Interesting ideas, especially the .357-.444, though it looks like it's tapered rather than necked, which is odd. Why not neck it down a la the .357-.44 Baine & Davis?
On that note, a while back, I had a notion that I never followed through on to neck a .45-70 down to take .35 caliber bullets.
As for fireforming cases out straight, the .270 Ren was a .22 Hornet that was "necked up" according to Hornady 6th Edition. Not sure if people actually ran thin Hornet brass through a sizing die and hoped for the best or if they fired .22 Hornet ammo in a Ren chamber.
I did once get to see the result of a .308 Winchester fired in a .270 Winchester chamber. Guy had two Ruger 77s that were identical except chambering, had switched guns and been firing the .270, but didn't take his .308 ammo off the table. He accidentally grabbed a .308 and chambered it. I wasn't there when he fired it, but he showed me the case. It was blown out straight save for the mouth, which looked like it was roll crimped, and was split in several places. I'm not sure if the splitting was from the pressure of shoving a .30 caliber bullet down a .27 caliber bore, from the mouth of the case meeting the shoulder of the chamber, or from the violence of expanding to the chamber.
If there's no problem with actually blowing the case out by firing the ammo in your straightened chamber, I would think the .30-30 would be fine. The .223 would present more of a problem. You've got to headspace it on something. The .30-30 can headspace on the rim. The .223 doesn't have a rim to headspace off of.