Some observations :

1- In those cases, I even have been able to fight a siege battle after gaining a warning repeatedly...

2-Sure is so...

3-Not sure on this point, if you're attacked in field battle by the faction you got a warning against, I've never seen excomm or loss of status happen...

4-That's the whole of bribing the Pope (and is not far from what was done in reality at the times : Popes were generally either puppets or puppeteers)...

As to when other factions get warnings, I think, but it has still to be proven that both warring factions get the same warning at the same time...

On this point, the Danes got excommed for continuing fighting me after I got a warning so I think the may have gotten one as well or maybe they wrer just fihgting on another front against another Catholic faction and got warned for that...(No savegame to check w/o FOW)...