hi
We do this a little bit but usually do more of a Model C and simply help underwrite the programs vs absorbing them as a Model A project. This does not actually solve the two issues you have but just wanted to give that context.
In general I think this comes down to an instance where the client cant "have their cake and eat it to." I think it also helps to think of the "costs" of Fiscal sponsorship as being more than just $$$.
"have their cake and eat it to."
They cant offload services and risks to you while still keeping all autonomy/control.
If they want to move all their programs under you then thats cool... that feels like a full Model A. I don't feel like they can keep their separate entity active AND have you be a sponsor over all their activities.
More than $$$
Projects "pay" an administration cost and in turn you provide support and systems.
Projects also "pay" the sponsor in handing over some level of control/autonomy over their operations. What they get in return is mitigated risks as you define what is right/wrong. As your appetite for risks goes down your need for more control over projects goes up and vice versa.
How much support they need/want --> Which model is the best fit --> how much autonomy/control they will need to hand over to you --> how much risk you can mitigate
So I think its important to ask yourself:
1. How much $$$ do we need to align with our budget? (i get this is not part of your question)
2. How much control do you need to "charge" to align with our appetite for risk?
2.1 - how much risk does the name "issue" really expose you to and how does that impact your overall appetite for risk?
I know this is not as much of an "answer" as it is simply some questions and thoughts... hope it helps in some way!
I know this is not really an anwser