No, PNaCl didn't use V8. It used an elaborate set of llvm changes that had problems and couldn't be upstreamed.

A big part of wasm's pitch is that it reuses JS engine infrastructure (less new engineering, less attack surface), but that means it's intrinsically more limited. No mmap, etc.

The V8 people didn't want to support heap growth at all (it made some part of the JIT harder) but eventually were convinced. Imagine having to pre-allocate 512mb or more of heap for every little app you deploy. (asm.js already had this limitation for the same reason)