Your problem with the scroll is actually a misleading message - the scroll of body switch is unwishable, just like a scroll of wishing is. The game only uses a confusing message.
As for the problem with polymorphing into a siren - this is because there are multiple types of sirens and the game doesn't know which one you want. It's actually the same problem as with controlled polymorphing into a skeleton or a ghost - you cannot simply polymorph into "skeleton" either, because they are randomly generated from other creatures, but the polycontrol engine does not understand this at the moment and requires you to be more precise.
You can circumvent this by using the polymoprh list instead - on the prompt, press "?" for a list of all creatures you can polymorph into, then select a siren. If you don't see a siren on the list, you have not yet encountered any, which is a requirement for normal polycontrol to be possible.