The same thing happens literally every a census indicates population change - there's no reason to think it would require renegotiation.

Representation isn't uniform and allocated per-capita. It's arrived at through negotiation, horse-trading and principles that seek to give a voice where it would otherwise be diminished if left to something as simple as population. The hit to Spain's territory, it's GDP and subsequent contribution to the central fund would undoubtedly trigger a re-negotiation.

EU representatives are not assigned to geographical regions in member states, so I don't know how you'd even make such a distinction.

You're totally wrong, they're called Constituencies and Catalonia has 4.

You realize that the EU is also bound by that same international law, yes?

Inter-between, National-Nations. The EU is not a nation. It's a product of International law-Treaties are instruments under International law-but it's internal workings are under the jurisdiction of EU law. They aren't bound by any of the principles of Intl law beyond that what pertains to the Treaties governing it's creation.

First of all, it's not a queue. There's no "ordering" of applicants.

Second and more presently, Catalonia is not an EU member country (largely by virtue of not being a country), and therefore cannot claim to already be one. In particular, Article 17 of the Vienna Convention on Succession of States in respect of Treaties explains that newly-independent countries can only declare themselves party to treaties the "parent" (for lack of a better word) country was party to that depend on unanimous consent of the parties if the newly-independent country also obtains unanimous consent. This would quite obviously include the EU treaties.

http://www.bbc.co.uk/news/world-europe-11283616

'In the Balkans the breakaway territory of Kosovo is last in the queue to join the EU... '

I know Catalonia isn't a country, that's self-evident, but it does have representatives, elected by EU citizens who can forward motions in Parliament.

I am not claiming Catalonia will inherit the rights and obligations of Spain should it declare Independence, what i am suggesting is that the route into the EU that you suggest, a Article 49 ascension, would be out of the question on practical and political grounds. This is real-politik, not some semantic stand-off.