This Summer youll also have to pay mandatory reservation to and out of germany. 3 Euros each when buyed with ÖBB. Or you buy the whole route with DB for 4.90. There is mandatory supplement on the Brenner ECs but only on the italian part. So in your case between Brennero and Bolzano. So you could avoid it by changing their to a regional train.
1.
Perfectly possible.
2.
There’s a supplement of 10 € 2nd class or 15 € 1st class when travelling with direct EC trains (possible to buy online with ÖBB). It’s possible to avoid this supplement by changing to Trenitalia regional trains in Brenner (border station; 80 minutes from Bozen) but of course less convenient. If you are travelling in summer (1.6. to 1.9.) there’s a mandatory reservation needed for the EC trains when crossing the border from Germany to Austria, too (3 € if bought with ÖBB).
3.
If you do avoid Eurostar between Antwerpen and Köln and you don’t travel in summer: yes, it is. Just in summer (1.6. to 1.9.), you’d need a mandatory reservaton for ICE trains between Belgium and Germany, too (3 € if bought with ÖBB).