Open Sans is almost indistinguishable from Helvetica. I just created two pages using each, and I could not find any real differences.
I don’t really remember enough about Weaver II to know the source of the Helvetica font it used, but there are copyright and other issues with using Helvetica. I think there was a crackdown, and Weaver Xtreme uses only public domain fonts. That is why Open Sans was created and is widely used.
Weave Xtreme Plus does allow you to upload whatever fonts you wish and add them manually. There are instructions at