Can half rounds be shortened successfully

No. But you may be able to use longer ones than the bass needs. It’s not ideal. What exactly are you trying to do?

 

Yes you can. I use D'Addario half rounds on my Hofner Beatle Bass. I could never find any short scale strings, so I cut down long scale. The only issue I had was having to fit a fatter part of the string through the tuning peg on the headstock. Just be careful once the string is cut, because if you don't take care the string can unravel... and don't cut them too short, you can't put it back together!

 

I'm confused why some people say you can't cut a bass string.
Why not? The string makes contact at the nut. Beyond that you're just putting a few winds around
the tuner and trimming to taste.

Is there some voodoo going on ?

 

Cutting a wound string in the part that's meant to be played (the "speaking part") can be problematic. Yes, sometimes you can just cut it, and all will be well. Sometimes, though, the outer winding can start to unravel - sometimes right before your eyes - or it can slip loose from the core wire. Or the string will do both... This is why string companies do what they do on the part of the string that's meant to be cut -so it won't do bad things when you cut the string there. You can usually - usually - avoid the potential problem, by putting a sharp bend in the string behind the spot where you want to cut it - before you cut it. Or, you can avoid the problem entirely, by using the correct length strings. Sometimes, though, you can't do that - as in the case of sub-short scale basses. Or if you have to, as an expedient thing; or, I suppose, if it just makes you happy...