The thing is that three rounds is still all you should ever need. If you need more then three rounds in one engagement, you're short of people on your side of the argument.
Shoot three, and if you still have any, strip them out with the bolt. It's foolproof enough that conscripts understand how it works.
Does anyone know what the soldiers of the time were actually trained to do? I think that this is more of a matter of what the actual soldier of the time would do, rather than if it was physically possible. What does it matter if you can dump the ammo out of a kar98 if all the soldiers were trained to never do so in battle. If there isn't any evidence of soldiers either being trained to use the mag dump in combat, or of them just using it in the field, regardless of training, I don't see the point of it being ingame.
As for topping off, I could see you just shoving a partial stripper clip in, and retaining it for later use.
I was more asking if soldiers were trained to use the mag dump on their firearms in battle, or if it was only for servicing the weapon.
I also think that riflemen should be able to manually eject unspent cartridges by working the bolt without firing a round.
This would make it so you could silently clear the mag to load a new stripper clip.
I think this would also be an easier solution to what the OP suggested, although that isn't a terrible idea, for example a alt-reload single feed option mapped to a key on the keyboard.
What i think would be the best for both mosin as k98.
When you hit the reload button you grab a stripper clip from your pouch. Then you insert it and push it down how far it goes. Then If your gun still had ammo inside you cant push it all the way in so you remove the stripperclip with still some bullets attached. But effectively topping off the rifle. And then put the half empty stripperclip back into your pouch at the end of the cue.
Word!And a seperate button for bolting might be taking it a step to far.
I wouldnt mind if one could bind a seperate button to cycle through the bolt. However remember that twi tries to minimize the buttons etc people have to learn to play the game. And a seperate button for bolting might be taking it a step to far.