You can have easily one point in intelligence through garment, and usually another one on collar (STR + INT combo). If you have consistent luck (I would not count on it much), you can also have INT on the body armor, but that one should be changed often to keep up with armor, so it's not likely you will always have INT bonuses on it.
I would not use lower willpower in witchcraft (there's already a man at arms skill for that, which is AOE to boot), nor soulsap (high INT requirement, high AP cost if low witchcraft level, low hit chance).
If your warriors do not end in precarious situation often in battle (near death), and especially if you don't play glass canon, you can stop focusing on constitution and focus speed more (that's where the AP regeneration comes from).
I don't have particular advice to give what is best to focus on, I personally blindly invested all in strength on my warriors, without considering much what's best at the time. I noticed there's huge return for INT (because it lowers cooldowns), but not so much for STR, so I don't know. I need to see for myself first what strength is good for exactly, and if more is always better.
