Another problem is that I believe that the MCBs I have in my CU are single only pole.
Not a problem.
If your MCB only switches L and omits the N connection, then that MCB is NOT an RCD/RCBO, and can be ignored insofar as measuring earth leakage is concerned.
I think there may be some confusion between both L+N physically passing through the RCD/RCBO, vs. 2-pole switching of both L+N.
Most RCBOs (at least in UK) only switch the Live, with Neutral just passing through for residual current measurement. This means that any neutral-earth leakage will still be there when it's turned off, and a N-E short on that circuit will trip any upstream RCD
Indeed. When I Googled the manufacturer of the breakers shown in the posted image above, the breakers I found only had a single physical screw terminal on the line and load sides.
So unless I Googled the incorrect model breakers, the ones shown in the posted image above CANNOT be RCD/RCBOs because they do not have both L+N.
In other words, RCD/RCBOs must have L+N physically passing through them. They may switch L only, or switch both L+N, but they MUST have both L+N physical connections.
I have attached a photo; this is NOT a RCD/RCBO.
In contrast this is a RCD/RCBO. Note the extra neutral connection. Also the big tell-tale is the TEST button that all RCD/RCBOs have.