Chromium supports a single process switch at least to use for development of Chromium itself. While it does reduce memory consumption, it does not help much. All data structures in Chromium are tailored for multi-process case with no memory sharing. Replacing processes with threads do not change that.

As for removing V8 JS engine from blink I guess it is possible. But again, blink is tailored for accessing from JS and the layout and rendering code is huge so one does not save much.