So, Polymorph subschool spells apparently end if you want them to, whenever you are subject to a new polymorph spell. Does this mean I can use literally any Polymorph spell to end Baleful Polymorph? Furthermore, since the decision to allow Baleful Polymorph to affect me or not is made by myself at the time the spell is cast, while the spell dispels other Polymorph effects when I fail to save, does this mean that being under the effects of literally any Polymorph spell (e.g. Youthful Appearance, which is a level 1 spell) makes me effectively immune to PaO? This would seem to render effectively impotent a high-level classic spell.
Support for the issue:
Any polymorph effects on the target are automatically dispelled when a target fails to resist the effects of baleful polymorph
You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you
The Spell's Result
Once you know which creatures (or objects or areas) are affected, and whether those creatures have made successful saving throws (if any were allowed), you can apply whatever results a spell entails.
So it would seem we get to choose not to allow Baleful Polymorph to affect us before we save against it. Even if we choose a weird all-inclusive definition of 'when X is cast on you' (which has other problems), the effects would then be resolving simultaneously, which is ill-defined, and furthermore:
In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph spell.
So we don't have to save in the first place, even if saving against a spell happens before casting starts, unless we're already 'Small or smaller' as determined by what we're being turned into.
as long as baleful polymorph remains in effect, the target cannot use other polymorph spells or effects to assume a new form.
If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell.
You can only be affected by one polymorph spell at a time.
So the situation this line refers to, where you have had some other polymorph effect added and still have Baleful Polymorph so it's limited to only its non-form-altering effects, can't actually occur ever with regards to spells! Even if we choose to keep Baleful polymorph, the other spell isn't just
unable [to allow the subject] to assume a new form.
it's completely negated! The polymorph spells cannot coexist. I guess this matters for stuff like Wild Shape, which Baleful Polymorph also prevents from altering your form, but which don't trivially negate Baleful Polymorph if you feel like it and automatically get dispelled if you don't, but that seems very contrary to the RAI, to me, which seem to be that Baleful Polymorph actually works regardless of other polymorph effects.