So when can a tx be considered unreversable ?
Never, look at formula #14 in
http://188.138.57.93/tangle.pdf. Just like in Bitcoin there is always a chance of doublespending.
Not even when the weight cap is reached ? (The paper mentions the cap but I'm not sure it ever states if the cap will actually be applied)
That cap is only for the own weight of a tx (in fact, as far as I know, the plan is to set it to constant). The cumulative weight may (and will) grow.