1) you could make Johan into a support mage--in fact that is what I usually have him do if I plan to have him as a regular party member. But a support mage should still specialize in a select number of schools of magic to maximize their potential, rather than spread their points around. I would suggest water/earth because those schools provide a lot of CC spells and "set up" spells that synergizes with combat spells from the other caster(s) in the party. I don't know if I would give Johan witchcraft beyond just a few points. Witchcraft does have a lot of buff and debuff spells, which works great for a support mage, but also has a lot of excellent close range spells, and I typically find it better to have a combat warrior/mage specialize in witchcraft to fully utilize its potential. No need to march my range mage into melee range of AI mobs to cast death punch when my warrior/mage hybrid is already there
2) willpower and bodybuilding can be useful if a character invests in it, but it's not really worth the effort IMHO in light of the limited availability of points and the soft level cap. It's much better to invest those points into your caster's spell schools and use spells to buff your front liner and counter enemy CC and debuffs (thereby shooting two birds with one stone).
3) I find shields rather limited in their utility. Presumably it would be useful to have a shield on the party member who is going to be the dedicated party tank, but that degrades their damage output potential in comparison to a warrior with a two hander. I find it much easier to use spells to buff the tank. If I need a tank just to soak up enemy damage without consideration of whether they will inflict any significant damage to the AI mobs, I usually just use summons as they serve that role quite well.
That said, while melee tank characters are sort of lackluster in the early game, they really starts to shine in the late game with enhanced gear (although crushing fist is always a great CC tool). Since you are playing on normal, feel free to experiment and go with whichever type of warrior template you prefer.
