[kta]

a. Are the encounters in general too easy? Too tough?

This depends to a great part on random enchantments some creatures have - I frequently encounter perma-hasted-immune-to-knockdown wereboars that put up a hard fight, or creatures that can't be harmed with the available weapons.
When I think of the Greater Werewolfs or Werewolf Lords - they put up quite a challenge as well and some of the blooded creatures too. While walking some dungeon alone and facing 2 Thayan Knights, a Red Wizard, a Apprentice and two Legionnaires and some chiefs at once is a no-win situation even for some rather powerful PC's.:P

c. For you with high-level PCs, are encounters a challenge at all anymore?

Depends on the place - take the Bezantur Crypt for example, the only encounters that scale with a PC after say lvl 10 are the mercs...

2. Creature selection. Thay uses mostly default NWN creatures in our spawns, with approximately 10% custom created (such Dread Warriors, Blooded Creatures, Spellstitched, etc). The variety of default creatures with a CR of 12 or greater really starts to drop off quickly, which I believe are making things too easy/boring for level 12+ PCs. So I ask;
a. Do you think Thay needs more creature varieties created?

Oh yes I would like that! And I have a suggestion how to... when you think of the PW's vampire system - why not vary the vampires a PC can encounter based on that system?
Instead of common vampires and a occasional vampire warrior we could have all kinds of vampires, Fledgelings, Masters, Elders even epics and so on... also wizards, rogues, clerics. You name it. This system could be adopted to virtualy all kinds of encounters.

b. If so, at what level(s) should these creatures be created?

Well, all levels could be affected but the mid to highs should be the first target.

3. Upper level PCs. I am fairly certain (95%) I will be temporarily capping PC levels at 15 in the next couple days until more upper-level creatures can be created which will provide more challenges to PCs above level 12+. Depending on whether or not any help is received, the cap may last only days, weeks, or potentially more than a month. When or if a cap is instituted, creature creation will become the top design priority, however any XP accummulation more than 1500 over what is required to get to level 15 will be lost. I know this probably won't be popular with PCs at or nearing this cap, but I want to be forthcoming with this as I truly believe you will have more fun if encounters are not so much grinding out the destruction of 100 skeleton warriors but instead engaging in a difficult struggle that uses most or all of your magic, strength, or tactics on 10 relatively difficult 'greater skeleton wariors' (for example).
Thoughts/comments?

Sounds good!

Another thing that can be used to make overland encounters seem more "real" and challenging: I always found it strange to meet a direwolf and a malar panther hunting in a "pack" instead why not create real dangerous wolfpacks or groups of "fitting" animals and make them more dangerous for higher lvls?

KTA