I think you make a confusion between two kinds of QUE : in the exemple you give (” je n’ avais aucune idée qu’il consommait…”), QUE is a relative conjunction, totally different to QUE in the following exemple :
“J’ai qu’une idée” ‘I have but one idea’, ‘I only have one idea’.
This second QUE is the one you want to discuss here.

Answer to your question 1 : I can’t think of any sentence where you can have both AUCUN ha QUE ‘but, only’, since AUCUN means ‘not even one’ and QUE means ‘just one’ (“Il n’y a qu’une personne qui…” ‘There is only one person who…’), or ‘just fifteen’ (“Il n’y a que quinze personnes qui…” ‘There are only fifteen persons who…’), or (etc.).

Question 2: You can say :
“Il n’y a PLUS QUE trois personnes qui…” ‘There are ONLY three people LEFT who…’
“Il n’y aura PLUS JAMAIS QUE trois personnes qui…” ‘There will ONLY EVER be BUT three people who…”