Soli Deo Gloria!
1) Does "March Debacle" card, affect a French client force such as the Swedes or Danes?
I would say not; just a French force. But this is just my opinion.
2) Can the Danes become a Belligerent French client, after France has been knocked out of the war?
This would only be possible if Britain rolled on the Danish Intervention table - and why they would do that once France is out I have no idea. But assuming they played "Wooing the Danes" and rolled poorly, I would still think that Denmark couldn't come in as a French client. Admittedly, the rules don't cover this exact situation specifically. However, once a power collapses, it and its clients are "immediately, permanently out of the game." (21.3.2) It would be strange if you could be out of the game but still be a patron to a minor power.
3) After France is Exhausted, they are limited to one Colonial OP per year. If they have already played Colonial OPS that year, then go exhausted, are they prohibited from playing cards until next year.
My understanding is that the prohibition is not on playing cards in general, just on playing cards for colonial ops. So they couldn't perform any more colonial ops that year, but they could still play cards for events and activations normally.
4) A force is 20 SPs strong, can a 5 SPs detach from this army during the minor force admin march phase? Or must a minor be less than 6 SPs from the beginning?
You can detach a minor force from a larger force as long as that force has not already activated or admin marched.
5)SITUATION: Prussia has 2 SAs, they move Leader A's force he picks up Leader B's force. Enters a enemy fortress space, drops off Leader B's force, continues moving on to a second enemy fortress. Can the Prussians lay TWO sieges this turn?
"Forces dropped off cannot expend any further MPs or set /prosecute sieges during this Strategy Card Segment." (12.5) So the answer is no.