Hmm...do you remember if the injury/disease thing happened after he got it from a potion? That could be something. I haven't seen it recently at all.
The negative gold thing: the 1.6 patch will officially make that change -> if you have no gold at the start of a new league day, you will lose the game!
And yes, warriors do leave if you don't have the funds to pay them their salary at the end of the contract phase. Basically if on payday at your job if they told you, "sorry, no money for you". I'm sure you'd tell them "sorry, no work for you".
The mouse popup on the end of contract phase button does warn you of this, as does the manual I believe as well.