Regarding the bus from Prague.
Yes, the bus is easier and faster and non-stop. It is a railway provided double decker coach with amenities. You can go by train to Nuremberg but it is slower and has many stops, and is nothing special.
The Global Pass is UNLIKELY to meet all your needs in the countries you have named. There are considerable extra costs.
French fast trains - the TGV - are rationed to pass holders and sometimes you won't be able to get on the one you want without paying a new full fare ticket for all travelers. You need to make those reservations just as soon as you can commit to the date. There is a not insignificant charge for each person on each different TGV train - so if you take a TGV and change to another to complete your journey and return each person will have 4 fees.
If you just buy French tickets ahead you can get very good deals (PREMs and a number of other discounts are available, and diminish the closer to travel) with no rationing and those reservation fees are included in (lower) price of the tickets.
Italian faster and fast trains (anything higher than a Regionale - coded R or RV) (which include ES - advertised as Freccia, EC - Eurocity, IC - Intercity) within Italy or cross border) all attract a non-rationed mandatory pass holder seat reservation fee of (at last look) €10 per person per leg on Freccia and €3 per person per leg on everything else. These reservations can be obtained until just before travel, if there are seats still on that train (usually unless a national holiday or rush hour) but it can be slow - at a window - or a little confusing and you often have to put up with "helpers" looking to make money if you use a machine.
It is possible to get advance tickets on many routes for as little as €9 per person - on the same trains - and they include the reservation fees, so not only is it cheaper than the pass holder reservation fee there's no pass day to pay for either.
Most people find that passes in Italy make little sense.
Passes in Italy are not valid at all on many trains.
Passes are not accepted on the trains around Milano operated by TreNord, including, to the Milan airport (Malpensa Express), and around Lake Como.
Passes are not accepted on the local trains around Napoli - Circumvesuviana to Sorrento and Pompei among others.
Passes are not accepted on many trains in the south of Italy.
Second Class passes are not accepted on the Leonardo Express from Roma Termini to the airport in Rome.
Passes are not accepted on the new national competing fast trains provided by Italo Treno. That's a real pity because they are lovely trains with excellent customer service and a very easy to use website.