When something is difficult to explain concisely, it's often an indication that the rules might be unnecessarily complicated.
That seems to be very much the case here, so I think the best answer would be for giffgaff to seriously consider whether it's absolutely necessary for the rules to be quite so complicated.
The 6 month rule primarily exists to comply with ofcom's requirement for number recycling -- a network must be able to demonstrate that the majority of its existing numbers are "in use" when it asks ofcom to allocate new numbers.
Ofcom doesn't care what method of payment is used to pay for a service. All it cares about is that the number is in use. So we could, potentially, simplify the rules without upsetting ofcom in any way.
The unwritten "new money" rule doesn't seem to be primarily a revenue protection issue either -- because one method of keeping the sim active is to make a single free 0800 call. It's hard to see how making an 0800 call has any greater financial benefit to giffgaff than buying a goodybag from existing credit, especially since giffgaff will be at least £6 up at the end of the goodybag month even if the goodybag was not used.
So, I'd suggest giffgaff looks in parallel at the possibility of abolishing the "new money" rule and making criterion d that the member buys a credit or goodybag, thereby officially dropping the currently-unwritten rule.