Sorry about tonnes of typos! Sent it from the phone without proof-reading.I think that's the only thing you can do. If you have no comnections with Cuba, you have to rely on the embassy. Even if you decided to travel to Cuba just for the PCC, there may be a mismatch with the dates on the PCC and the date that you leave (privided that the PCC has from and to dates, I've no idea what it looks like). I hope that IRCC are aware of the situation. Clearly, applying through different embassies makes no difference. The LoE doesn't have to have a specific format. It's saved on my husband's work computer, so can forward it later if you still need it. But just explain your situation in your own words. Say that you have done your best to obtain the PCC from Cuba, thay you have been to the embassy and applied for it (receip attached), say exactly what the embassy advised you in terms of the length of the process, attach any written correspondence if you have any. Also say that you'll provided it by CSE should it arrive earlier than expected, but in the meantime could they please waive the requirement and continue processing your application. Maybe also mention at some point that neither of you have had any criminal records in any country where you've ever resided in and that it proveb by the PCC that you were able to obtain.
Wishing you best of luck. Keep me updated and I will let you know about our progress too.
Also, remember to translate and notarize the receipt. I imagine yours is in Spanish, same as ours. Don't get caught out on something simple like that. Good luck!