question about "lexicalisation"

It's not saying that it cannot be done, just that it would be difficult.

Yes, you are right.

IMO Not quite. 'Words' are available to the speaker as part of the historical (diachronic) evolution of the whole language, but the speaker may be using an idiolect which itself is in the process of (synchronically) altering the meaning/usage of those words or even inventing neologisms. (E.g. take the word 'wicked' which has come to mean 'good' in some contexts, and may even come to be the standard meaning.) So the question boils down where the cutoff point is taken along the time scale which separates diachronic from synchronic. Note that Shakespeare was a one man lexicon production machine over his lifetime !

It can be and usually is. Take the words 'will' and 'going' as markers of the future.