You know, I'm not actually entirely sure if it sees the modifier as a valid selection, but yes, it will kill a modifier and cast a non-modified spell if it does not have the capacity. The only time it will not fire at all is when 1) the base spell has a greater cost than total capacity. Or 2) you are out of casts for spells that have a cast limit (donated by a number within the spell's block).
A shuffle wand will always cast all spells available in the selection before hitting the recharge, I can say that for sure. So if you have 5 spells each at 20% chance, then you cast one, you have a 25% for each of the remaining four, then 33%, then 50% then you will 100% cast the spell you haven't cast yet before the wand goes through recharge and all spells become available again.
When talking about dual casts and tri-casts I'm not entirely sure how it resolves what to cast when mana capacity is an issue, but I believe it goes left to right, and checks the validity against your capacity, yes=cast no=not cast which would be why someone would end up casting a couple, but not all of, a block's spells.
Spells and modifiers all can add (or remove) mana, modify cast delay/recharge delay. So do keep an eye on how that all gets changed based on what spells you use. One spell selection could be the difference between chaingun or a slow firing one shot. Some spells have 1 second delays built in (which technically can be overcome)