No 421 time, but I have about 1000 hours in a 402 (same plane, just unpressurized with non-geared engines).
The tail is very prone to stalling in the flare with a load of ice. Don't try to squeak one in if you have a load of ice back there. Just land flat and call it good.
Exercising the props at altitude with passengers...no biggie. Done it all winter. Just be smooth about it.