Well done for reading the article and inspecting.
By the time you actually see mites, it is way too late. You need to do proper mite counts using a sugar roll or alcohol wash method. If you use the Search tool at the top right (magnifying glass), you will find several links to help you. Here is one of them:
Most people treat at least twice year. If you can get oxalic acid in Portugal, I would strongly suggest using that to treat. It is cheap, highly effective and mites do not develop resistance to it. I use Randy Oliver’s sponge deliver method:
I only have a Flow hive classic at the moment, which doesn’t have ventilation options. If the vents are closed on your hive right now, I would leave them that way.
I don’t what you mean by “reversible hive”. If you have the plastic frame Flow super on top, I would take it off unless there are a lot of bees in it. It should not stay on over winter in your climate, because bees will put propolis in it which will gum up the Flow mechanism.
I would just leave them be. If they don’t have any honey stores, you could try feeding them. They probably won’t make it for much longer, but no harm in letting them try until your nucleus arrives.