I don't see personally a problem with a nominal stake, because English contract law only requires that something of tangible value, satisfying both parties, changes hands in both directions for a legal contract. There is nothing to stop me selling you my house or one of my aeroplanes for £1 so long as I am satisfied that this was fair exchange. Nobody but the parties to that contract are, so far as I understand, considered qualified to judge what constitutes fair value.
I believe that you are essentially correct, save and except for tax issues (as Gertrude pointed out), and fraudulent conveyances intended to place one's assets beyond the reach of creditors. Such statutory exceptions aside, when considering whether a contract is binding at common law, the courts are concerned with whether there is consideration and not with its adequacy. But in many common law jurisdictions, mere nominal consideration may not be enforceable if it is considered a pretense or sham transaction. See generally
"The Peppercorn Reconsidered".
Where's Flying Lawyer when you need him?
Alternatively, I am sure that FNG could provide comprehensive and reliable advice. Unfortunately, he is no longer with us, through no fault of his own.
is there not a requirement somewhere in the regulations for a jointly-owned aircraft to be owned in equal shares?
First I've heard of it. I don't see the point (not to imply that bureaucrats always have a point for their regulations).