Should EVERYBODY in that tavern get a save just by virtue of being in the presence of your illused self?
This I already answered. No, but the chances of not being discovered in that tavern are so remote that they'd might as well.
I tried it once and spammed the entire server until the spell wore off since the announcements all went out on the Shout channel. The spell needs some fine tuning me'thinks.
Write up a proposal and script it. Cipher has a fine system for tech changes in NWN1.
Useless 5th level spells still seems off to me.
It's not useless in every situation. It's useless in large crowds. Should one want to use seeming (which is just repeated castings of a level 1 spell, anyway) to fool two or three people, you're well within the range of the spell, and you've even got a pretty good chance of it working (Save DCs on seeming being in the 18-24 range, depending on spells, gear, feats, etc.) Got a right fine chance of getting past a blockade or walking into city gates with the spell.
However, there are plenty of 5th spells that are downright useless somewhere. (coincidentally, most spells are useless in a crowded tavern, unless you'd like to kill the people inside.) I don't see "lots of people around" being at all an unusual situation for a spell to be nearly useless, given the situations when other spells don't work:
Fifth Level Sorc/Wiz Spells That Are Useless In A Situation
Break Enchantment - what if no one's cursed?
Dismissal - what if there are not outsiders about?
Mage's Private Sanctum - this one would suck in a crowded place, too, what with people being able to just walk in.
Cloudkill - doesn't work underwater, and does squat vs. dwarves
Invisible Watchdog - if you need personal protection and move
Major Creation - if you don't have ranks in craft
Lesser Planar Binding - if you fail your charisma check, if you don't cast dimensional anchor fast enough, or if the circle of protection is disturbed. (of course, the outsider is likely to kill you if it makes its save)
Wall of Stone - if airborne
Prying Eyes - in the dark
Feeblemind - vs. undead, constructs, vermin (and generally nerfed vs. non-spellcasters)
Hold Monster - vs. undead, constructs, vermin, dragons, elementals, or oozes
Mind Fog - vs. undead. constructs, and vermin
Symbol of Sleep - vs. undead, constructs, vermin, dragons, elementals, oozes, and elves
Cone of Cold - vs. anything with the cold subtype
Mirage Arcana* - vs. lots of people
Nightmare* - vs. undead, constructs, and vermin
Persistent Image* - vs. lots of people
Seeming* - vs. lots of people
Blight - vs. anything but plants
Animal Growth - if there are no friendly animals about
Baleful Polymorph - vs. oozes, plants, shapechangers, and anyone with polymorph + silent/still spell
Transmute Rock To Mud - in any non-stone building
Transmute Mud to Rock - indoors
Waves of Fatigue - vs. constructs and undead
Fifth Level Spells That Are Easy As Fsck To Counter
Summon Monster V - most temples are warded against outsiders of opposing doctorines, preventing a summon of that kind from entering. Circle of Protection vs. Alignment would have the effect, were all of our summons not dire animals.
Teleport - dimensional anchor!
Dominate Person - protection from alignment
Magic Jar - protection from alignment
Fifth Level Spells That Are Actually Consistently Useful
Contact Other Plane
Telepathy Bond
Bigby's Interposing Hand
Sending
Wall of Force
Dream*
False Vision*
Shadow Evocation* (by the time they get to saying "Dude, that was an illusion," those who failed their will saves have already been pwned)
Symbol of Pain (unless you're a good guy... then you're gonna take some alignment hits here)
Fabricate
Overland Flight
Passwall
Telekinesis
Permanency
Illusion spells have been asterisked for easy finding.