Acho que você está pensando demais nisso. No entanto, eu acredito que você acerta quando diz
I think a much more reasonable interpretation is that I'll be allowed a laptop within its proper backpack/bag, plus a carry-on as described above.
Eu também penso em continuar assim:
You are allocated two slots for carry on's. One slot is in the overhead bin and the other slot is under the seat in front of you. As long as your carry ons can be accommodated in your allocated slots, then you are OK.
However the size of overhead bins depends on the aircraft in question, so for a smaller plane you may need to gate check your overhead bin carry on if it is too large for that particular aircraft.
Also you may not have a seat in front of you, in which case both cary ons have to go in the overhead bins.
Finally, cabin staff are aware of all the potential issues and will try and accommodate you as best they can.
É claro que, se houver espaço (ou você for o primeiro no avião), poderá colocar suas malas onde quiser, e o espaço reduzido se tornará o problema dos passageiros que se aproximarem de você.