The situational penalty is the penalty from sustaining the spell. You cast the spell. Spend a complex action to activate the power. And hey presto, the penalty is negated.
It doesn't just apply to the next action, it applies to the penalty - which lasts as long as the spell lasts, so the power lasts that long as well.
As to drain, noone is spamming spells continuously since combat usually doesn't last long enough for that to be neccesary. But yes, if you cast a lot of spells, you're going to get drain occasionally - that's the point of drain! Remember, not resisting all the drain isn't always neccesary - getting a few points of stun damage every once in a while is survivable. There's ways to deal with it in the short term (stim patches) and they don't last too long. Occasionally you roll really bad in a situation where it does matter - which is why you have Edge.
Lastly, relatively low-power characters (IE, fresh out of chargen) are supposed to have drawbacks. You can get to around 13 there - enough to more-or-less safely cast drain 3 spells, and more than even odds to cast drain 4 spells. Higher drain spells are likely to make you take a hit, but they're probably not that big a deal.
After your first two initiations - a priority for many runners - you can get Channeling and get that to 15. A channeling focus will get it higher - to say, 18, and you can get an increase attribute spell (for willpower or your main spell) for 22. That's 1 spell to sustain, 29 karma in initiation and 9 karma and some nuyen in the focus. That's a few missions worth, sure, but certainly not prime runner territory yet. And that gives you 22 dice. Enough to cast drain 5 spells with relative impunity, and enough to have more then even odds to ignore drain 7 spells.
