My subfloor doesn't meet Kährs' criteria as it's hollowed. How can I rectify this?

The easiest way is to put several layers of cardboard underlay in the hollow, until you get an even transition. Also, if you're using plastic sheeting, make sure that the underlay is on top.
If it's underneath, it could become mouldy.

Can I lay my wood floor on top of a plastic/vinyl carpet?

In most cases, yes. Also, should the installation require plastic sheeting, it won't be necessary as a plastic carpet, in most cases, is even denser than sheeting. Just use a step noise reducing underlay directly on the carpet.

Can I lay my wood floor directly on a textile wall-to-wall carpet?

No. The only soft carpet that you can lay a wood floor on is needle felt carpet or similar hard material. If you lay a wood floor on a soft carpet, there's a risk that the glued joints will break; this is also the case with any subfloor that is too soft. NB: If the installation demands plastic sheeting, it must not be laid on top of an old carpet due to the risk of moulding. Also, consider the risk of dust mites in an old textile carpet. In most cases, you should rip up the carpet. If you're unsure, contact Kährs for advice.