Cubana was candid alright. They just forgot to mention that they got the airplanes in a complex barter deal (read: for no money) from Russia, and after Russian invasion of Ukraine in 2014, Russians failed to keep the flow of (free) spare parts active. Running a fleet with no spare parts, expired airworthiness paperwork, and no OEM support is indeed complicated (and not necessarily safe), and grounding was a good idea. Even better idea was, as you mentioned, to pick up the phone and call the actual OEM, i.e Antonov. And hallelujah, somebody actually did it last year!
OEM and Cuba found understanding, hands were shaken, and apparently, some money even changed hands, to buy spare parts, get technicians to spend time on the frames, and get the fleet into the air again:https://antonov.com/en/article/cubana_de_aviacion_news
Free or not, the An-158 does have severe cracks in the fuselage, and there is no easy fix for that. Now that Antonov is trying to cozy up to Boeing Cubana can forget of ever getting parts from Ukraine.
Wasn't the An-148 built also in Russia? Isn't some way the MRO can be done through Russia?
1) An-148 was built as well in Russia. True. But An-158 wasn't
2) MRO might be possible, if there is a certified shop in Russia, with An-158 paperwork in order. Is there any?
3) Airworthiness paperwork has expired. Type Certificate holder (or someone, entrusted with relevant authority by TC holder) has to extend it, I would gather.
All of that notwithstanding, of course, Russia can always try to pull another "Volga-Dnepr An-124 airworthiness extension" stunt, and create a "local authority", and "vest" it with the "rights" to IP (that they don't own). Wouldn't be surprising, at all.