Ham789, Thanks for the precious comments.
I saw many HRV with similar ports positions, some of them do this to use a single motor instead of two, I wasn't aware they'd loose that much efficiency that way.If I got this right it would be enough switching positions between the motor n12 and the plenum n.11, right?
You are also right about the freezing issues but as I said that's why I'll use the pre heating exchanger. If only I could think of a system that regulates the heat only to prevent hxs from freezing, something like an electronic valve attached to Arduino... But that's as far as I can think for now.
@piwoslaw: thanx for cleaning up my post's mess