Mikey, if you can still find a copy of Dean Grennell's "Book of the 45,", he spends most of a chapter on the Marlin Camp Carbine, and was quite fond of what they could accomplish, within their limitations. The action is a straight blowback, hence extra-power loads are a big no-no. But given reasonable factory-level loads, you should be in good stead. Grennell never mentioned the "vigorous" ejection, that I remember, but he was also known to use a net on a frame next to his shooting position, so he may not have been bothered by having to hunt down the empties.
As for the spring situation, I'd be willing to bet you could find something with a little more oomph, Gun Parts Corp carries about a zillion springs, you might be able to substitute with something from an M-1 Carbine, or something similar. Don't unload it or pass on what might be a neat toy until you give it a chance. Grennell said that both the 9mm and 45ACP versions were absolute tack-drivers with ammo they liked, numerous sub 1-inch groups. Try it, you might like it! If not, there are a lot of folks who would love to play with one, they're hard to find!
Papajohn