I haven't tinkered with the editor for a long time and I was never an expert on combat, so you'll get better insights from someone else, but let me start off with the most basic of advice.
1) If you want to lock the party out of specific areas before they reach a certain goal, don't use locked or wizard-locked doors (which can be bashed or opened with a spell at any time). Instead, use doors with keys which give you control over when the party will gain access to them by finding the required keys. Or events that make the party take a step back until they've met a certain condition. I've hardly ever used simple locked doors and never used wizard-locked doors. They're quite useless for obstructing the players' path, IMO they're only good for show, making the players feel strong or smart for memorizing the right spell. You also get unlimited tries on bashing them, if you step away and then come back again to try again.
2) I'm not good at designing combat events, but my first thoughts here are: Have you tried letting the opponents start from further away and surprise the party? If the party gets to make the first move and the spellcasters are in close range, of course they won't get a chance to do their magic. Also, did you put the fighters and the spellcasters in the same combat event or were those two different ones? Spellcasters need tanks to distract the party, single casters won't stand a chance on their own against a party with melee fighters. And how many 5th level fighters did you let the party fight at once? Even if the PCs are weaker in a direct one-on-one comparison, there is power in numbers. (That being said, "Slay Living" used against a party of low level characters doesn't sound like much fun from a player's perspective, but maybe that's just me.)
3) No clue, sorry. I've never worried about this much, but I bet it is a pet peeves of many design authors, so hopefully someone else will step in here.