Maybe it will sound strange, but what is this task REALLY about? I mean - I can see two problems here, but no clear information about which problem is a real problem and - if it is - what is the expected behavior.

Problems I can see are:
1) Type of returned value changes depending on the value source (list for user provided or list for default)
2) It's impossible to set list as 'default'

I understand that this task concentrates on 1st one, however changing behavior described in 2nd would - in my oppinion - fix 1st one too - am I right? User would "define" the returned type by defining the 'default' as a list or string. 

But - if we assume that we only discuss 1st problem here - what is the expected behavior? Provided workaround is suggesting that we expect string, but - basing on current nargs behavior and "intuition" - I'd rather expect to get a list, when I define nargs with "*" or "+". This sounds "natural" for me.

Could someone explain me the problem and the expected behavior in a clear way?

Sorry if I'm not clear, but it's my first post here and maybe I've missed something in the description or I assume something that is not true (especially that the task is quite old... ;) ).