[...] Upon firing only one round and reloading, the spent casing of the fired round doesn't get ejected and we're left to assume that it just vanished. I don't know how you would go around to addressing this, given the strange nature of the hybrid shotgun, I don't even fully grasp how it's supposed to work. You could make reloading only possible when both chambers are fired and cleared. [...]
Uh, that's true. And all of your suggestions are valid, but I have another (quickier) idea to fix it: when you fire an odd number of shells and then reload, you might have the pumping at the end of the reload sequence to eject that one last shell you still had in the chamber - the shotgun refill system is supposed to inject a new shell where the old one was ejected from. What do you think about it?

[...] First, I suggest you replace the "Already carried" notification with "Already carrying". It's more grammatically sound.
Second, getting the "Already carried" message, obviously by walking over weapons you already own, spams too many repeated message in the console. Could it be made so that the notification is only displayed on the console once, or if that's not possible, then just never? And preferably without the extra red lines too, or whatever they are, because those take up added space in the console. [...]
I second the grammatical correction! Don't know why I didn't realize about it in the first place...

About the messages high frequency, I might try to increase the tics duration of the pickup actor blank frame where the message display function is called...
And the red lines in the console, I suppose they are due to the kind of DECORATE call - don't know if they appear for A_Log too, or only for A_Print, but I guess that reducing their frequency would solve this problem, too...
And last but not least:
thanks for all the feedback!

To me, it's really cool that you and others liked the mod so much.