This is a well-understood principle the EU holds regardless - “no negotiations without notification” is well understood by all parties and not somehow a new thing dreamt up for Brexit. And it makes good sense - negotiating with someone within the EU is very different from negotiating with someone outside of it or someone leaving it, and tipping your hand early on that is disadvantageous for the 27 remaining EU members. Is it unreasonable for the EU to not willingly give up an advantage in negotiations with the UK? No - it’d be irresponsible of it towards its own members to do so.
While the previous bit was at least somewhat reasonable, this is a farce. The EU and UK spent two years working out a deal in what was allegedly good faith. Given the incredibly tight time limit and the extenuating circumstances (such as the Good Friday agreement), this was already a very heavy lift. Given that, the expectation was that this was the deal that both countries would accept. Then, with very little time the UK voted it down and expected a new, better deal to spring out of someone’s ass. To say that this is a diplomatic faux pas is putting it mildly. If the UK didn’t want that deal, what did it want?
(This continues to be a sticking point in negotiations, by the way - the UK doesn’t seem to have a damned clue what it wants, and when they mention what they want, what they want is typically less “things they can get” and more “everyone gets their own unicorn” requests like full participation in borderless trade without participation in free movement.)
Under those circumstances, the refusal to renegotiate again is not unreasonable. In fact, demanding another attempt at negotiation with such a small timespan is unreasonable, bordering on absurd.
Let’s be clear here - yet another extension not being quite as long as the UK wants is not “the EU being unreasonable”. It is “the EU bending over backwards but not quite being willing to touch the ground with their nose”. By this point in the negotiations the UK had burned through just about all the goodwill they had had with the EU. They had consistently acted in bad faith. The UK requesting another extension to do… something? It’s just time-wasting. Shit or get off the pot.
The EU has bent over backwards to be fair and reasonable to the UK. It has not, in spite of the accusations of certain bad-faith actors within the UK, been “unreasonable”. The UK wanted to leave, and Europe, as it is obligated to do so, allowed it to. Care to outline what the EU should have done, proactively, to “keep the UK”, that wouldn’t have amounted to privileging a leaving member for leaving, thus allowing for some very unreasonable hostage-esque situations?
The examples you offer are extremely small potatoes. The major procedural hurdles involve things like “demanding a new major trade deal within two years” and “not knowing what the hell they want”.