Thanks for the clarification on several points, much appreciated!

Eventually, after contacting Iberia Plus via the UK number twice and having no success, I called the Spanish call centre number (not sure if they route to the same group) and spoke to a lady that did indeed confirm that I was able to redeem Iberia Plus Avios on Avianca flights. She did indicate that you can only book return flights via this method, and also due to recent changes was unable to confirm how many Avios I would need to redeem. However, she has promised that someone would call me back with the details.

I'll browse through the Avianca section to see if I can potentially figure out whether there are seats available.