Skaven Questions ... and Irresistible Force

Collected from various parts of the interwebs.  These are the questions that I think will generate the most heated debate between Skaven players and their opponents.

• The Dreaded 13th Spell: Can the spell be cast on infantry units that house larger models (such as Slann with guard or the clanrat unit pushing a Screaming Bell)?  If so, what happens to the differing model if there enough transformations to cover the entire unit?
• Banner of Verminous Scurrying: When does the triple paced movement conferred by the banner take place?
• Warlord War-Litter: What size base should the war-litter use?  Can the war-litter join units?  If so, does it count as two models wide for purposes of rank width?  What is its unit strength?
• Are Warpstone Tokens really not "one use only"?  If they are, why give characters multiple tokens to start?
• Can plague priests use warpstone tokens?
  
• Is the Storm Banner really not "one use only"?
• Are Master Moulders required to be in the front rank of a Rat Ogre unit, since they are in all respects a unit champion?  If so, how does the mix of 20mm and 40mm bases affect ranks?
• When adding packmasters to a unit of  giant rats / rat ogres, can an additional packmaster be added to a unit 5 rats / 2 rat ogres or does that unit have to be 10 rats / 4 rat ogres before a second packmaster can be added?
  

On another note, I learned something new as a result of the 13th spell.  To successfully cast a spell you need to roll either greater than or equal to the casting cost or cast with irresistible force.  As much as we've tried we cannot find a rule that states you need to throw enough dice to make the casting cost even feasible, so it would be perfectly legal to cast the dreaded 13th every turn with two or three dice, just on the chance that you'd get IF.