How easily/well do Euros (rather than USD) trade in the cuevas? Will I take a significant hit in exchange rate terms in selling euros rather than US dollars and would I be better off changing my euros to US dollars and take these to Argentina instead?

My exchanging euros to dollars first here will typically entail an additional 2.5-3% hit in terms of exchange rate loss so if the "premium" to be paid in spreads for EUR/ARS rather than USD/ARS is not more than 1 or 2%, I would probably be better off taking euros. If the premium to be paid is more (or if it is significantly more hassle to find a cueva accepting euros), then the double conversion EUR>USD>ARS would still be worth it.