To construct a skin boat I guess you would need a fair degree of carpentry skills, keel constuction, ribs and decking etc
I reckon stone tools could do the job, I guess those folk in the Neolithic were at the pinacle of European stone tool technology.
"Would flint really have been effective"
Hell yes! a freshly struck blade can be sharper than surgical steel.
"is there evidence of well shaped and trimmed Neolithic planks from elsewhere?"
Off the top of my head - the Sweet Track and the neolithic causeways at Flag Fen I guess would show that planking was achievable and effective.
Also I would cite long houses as evidence of large scale effective timber construction.
I guess there is also ethnographic evidence from cultures that until recently have had to rely and stone tools and have managed to build wooden boats without too much trouble.

I agree with you on the question of when. I'm sticking with the Neolithic. To me that was a time when mankind, in Europe, took huge leaps forward in construction technologies

I guess we're all on the same journey and it's debates like this that keep me coming back to TMA.