you are right, with the 12 cards we can trade in, we would have -5 pendulums to the former vault.
since we get 30 cards now from the propaganda vote (yay!), we can get us the 5 pendulums/pillars from there.
(and we have still 3 unused fire pendulums in the vault).
so, yes i agree with discarding 48 pendulums.
edit:
what do we take from the propaganda (30 cards)?
a first suggestion:
5 life pendulums (or pillars, quanta bash worked too good vs. life in round 1?)
5 graboids
6 twin universe (or less? 4?)
3 quintessence
4 minor vamps
7 life critters (4 cockatrice 3 frogs?)
other posibillities:
4 immolation (to support the fire decks + probably rainbows)
more novas
rewind, ev. procrastination, dune scorps (would need monumentum and novas) for a time stall with dials?
enchant artifact