Bold 1: Mostly due to the obsessive spending pattern being imposed on the KM from Berlin, "build me big, impressive warships"...It was never 9 at the same time but over several years with only a couple of ships active at a given time - this is why they were so hard to find
Also the KM struggled to arm and crew them - which is why they were 'drip fed' into the war as raiders
With more ships - 'individually' they are more likely to be found and neutralized.
Certainly they would have a bigger impact but with multiple Raiders being discovered and sunk / scuttled - each one becomes a propaganda boost to the Allies.
OTL in 1939 - I think the British sank a dozen or so Uboats and the Graf Spee (and hounded German Merchant shipping from the high seas) - here more victories are added with little extra effort from the 100+ Allied Cruisers hunting them
And if the Allies - particularly the British get wind of this plan before 1939.......well it only has one target in mind.
Bold 2: Only due to the fact that they were directing manning requirements on a different tactical axis.
Bold 3: This is the fundamental advantage here. The oceans are HUGE. If the KM have 4 x the historical Hilfskreuzer fleet at sea when things "kick-off" in September of 1939?
Bold 4: This serves to elaborate on my point made in #3. The RN did not have the requisite manpower/assets to go off hunting down vessels like these. Have you any recollection of the "Destroyers for Bases" deal that the UK made with FDR in 1940? This was in large part to provide an immediate "surge" in RN strength on the North Atlantic Convoy routes, due to the U-Boat threat. They were already over-taxed with this commitment at the time.
The RN is going to have it's hands full trying to deal with the threat. There is a huge aspect of diffusion in play here and the advantage lies with the antagonist, not the defender.
Bold 5: The beauty of this plan is that all of this can be done on a clandestine basis. There are no overt moves implicit. They are moving 10,000 ton merchants through the "refit" process.
It's not going to raise an undue level of suspicion at the Admiralty level.
Until the situation goes HOT.
At that point? All bets are off.