Do you want it in french or english?
Not only have I already explained the Twisted Spire thing, it's the same behaviiur as vanilla. It is a little counterintuitive, mind.
Re: Negative healing, if I remember the healing algorithm from the excellent article about it, units are only eligible for healing if they're wounded, haven't moved etc. - removing the first check might make a net negative heal rate actually drain the health of healthy units. However, the same argument would also say that you'd stay at the same health if you move even if you'd lose health when standing still, which would be more complicated to fix - might need to calculate the heal rate first somehow, then make the unit eligible if it's less than zero, then apply healing. Haven't tested whether that does indeed happen, but I expect I'm right. If you were fixing up that code, an improvement to the "turns to heal" display when heal rate is less than or equal to 0 would be nice - it currently overflows and gives some long number.
Actually, could you make gaining the Wither/similar promotion deal 1hp's worth of damage? That ought to guarantee healing eligibility and improve its effectiveness. At present it still works well with a collateral damage attack - wither everyone, then attack with a catapult or fireball to make it kick in, although heal rates in (non-rioting) cities are at least 20%, so it'd merely slow healing down.
On a related note, it might be worth noting that a friendly city is better for healing than one of your own which is rioting (assuming the same improvements, at least).
