Would you be willing to answer a few brief questions about PlayStation Forums?
They can, unless you mean directly from the PS Store... which I am not sure about. However, you can redeem a voucher from a sub account just fine.
Work fine for me... 30 day PS Plus codes and 7 Day Trials don't, but 1 Year work just fine.
IGN just opened an article asking for people to submit information of issues with companies and they will help you get an answer back. The sub account issue seems like a great one to submit! http://ca.ign.com/articles/2014/01/22/ign-has-your-back