Answer to some of your questions. 

You can only travel in you country of residence on two days during the validity of your global pass. If you want to go back to your country of residence between your trips you will have to buy a ticket for the travels in your country of residence. 

There is no difference in which trains you can use with a global pass and a one country pass. As far as I know more or less all trains in the countries you want to visit are included in the pass.

Yes, you can take the high speed trains, but all of them, except for the ICE in Germany, require that you make a seat reservation that you pay for.

On the TGV there is a limited number of seats that can be reserved by travellers with Interrail/Eurail.

You can plan your journey with bahn.com to have an idea wich trains run, if you get stuck somewhere don’t hesitate to ask here. 

In BE the NMBS offers pretty cheap junior tix-you should know even better as me-but never to Border points-only last stop in la Belgiqsque.

So spread out it seems to me a pass will hardly save money-unless you also want to make many more local trips in those cities you visit. But you really have to do the homework yourself-just check-spend an hour behind your computer-what fixed advacne tickets would cost. But then you are tied in to those trains/days and changing plans later will cost a lot of money.

NOTE/suggestion; IF you can use only the slow LOCAL/regional trains in DE (and not the fast white ICE/IC) there will be a monthly ticket for just 9€ for jun/jul/aug-it also covers all local bustram etc. But to use this for a ride S->B will at least double the time needed and many more changes. There is also a cheap low-budget train FLIXtrain=like the green buses. Does NOT accept passes.

Local trains can plan on bahn.de and set to ´nur nahverkehr´ (sorry wallon, I am part german so dont do francais)

Last yrs there were also such monthly tickets for local TER trains anywhere in FR during summer school hols there-29€ or so-but to reach Rns from BE on such trains is hardly possible. No idea if they will also be there this summer. (a bit like GO-unlimited on NMBS)