Ibn Ezra’s pshat in Raamses is one of several. This question was discussed on the leining list serve. I can check the archives if you like.
Re teven/taven: same as above. There are some extremely knowledgable daykanim in the group and some of the posts need to be learned like tshuvos rather than just read. There was some kind of obscure rule cited in RaDaK’s michlol that explained why teven didn’t change.