I think that it’s pretty safe to say that this piece is in ternary form. The A section ends in measure 15 on a PAC in the dominant of the original key. I’d say that the A section is a parallel period…but it’s assymetric. (did I spell that right?)
The B section, I think, can only be characterized as a transitional section. It’s very tonally ambigious tonicizing a couple different keys. It’s not really even organized enough to put into phrases or periods.
The A section returns around measure 29 after a PAC in the original key at the end of the B section. Although the A section technically returns…I wouldn’t say it’s a period like the original one. I’d say there are four cadences HC, IAC, PAC, and then a PAC with a cadential extension at the end. So I’d have to call this a phrase group because it’s not related enough to be a period.
Does that happen a lot in ternary form, by the way? I mean can you change the phrase structure of the A section and still technically have it be the A section?