The French garrison will remain in place until relieved by the incoming German administration, there will be no power vacuum for Japan to invade into.
Oh with regard to Indochina I was more thinking of the long term. German Indochina will be a geopolitical island in a sea of (potential) enemies. The UK will stay a sworn enemy and the Netherlands will try to keep it's neutrality while French Indochina could count (to a degree) on support of the UK and the US in case of open war with Japan. If the Japanese decide to have a go at the Germans and manage to take Indochina where is Germany going to stage a counterinvasion from? Ponchiderrry? Dar es Salaam? A transfer of power is bound to trigger Ho Chi Minh and the Vietminh as well.
I imagine that Germany would have no shortage of troops available in 1918 to garrison these colonies given the size of the Heer and Marines.
The Germans are in need of two types of manpower for their newfound colonial empire: military forces and administrators.
The size of it's military is indeed huge but the great majority are conscripts and after four brutal years of warfare the public will want to have their boys home, not on the other side of the globe. What was the German law regarding conscripts OTL? I know that the Dutch constitution for example forbade the use of army conscripts outside of the Netherlands. I'm not saying that the Germans will have a problem garrisoning Mittelafrika but won't be as easy as it looks like at first glace. See also the problems the Entente had with sending forces to Russia after WWI was over.
A bigger problem will be in finding enough administrators though. The German colonial empire before WWI was pretty young small sized . There is no huge pool of experienced colonial administrators to draw from. They could try to use Belgian/French/British administrators in place but the great majority of them will not want to work for the
Boch. The same with the colonial units, it will be hard to get them to transfer their loyalty.
Please don't see this as criticism to your ATL or the ATL peace though! I pretty much agree with your choices. I just think that the German (colonial) Empire will be less of a land of milk and honey as the Germans expected
