It seems after all this time, shifters are still the unwanted stepchildren.
Here's a list of feats/abilities the shifters use, whose DC isn't going beyond 25. I've omitted the ones that don't need to be increased. All of these were tested with a level 54 Shifter, so 37 Shifter levels (10 druid, 30 shifter build)
Rakshasa Ice Storm -- NW_S0_IceStorm.nss (Shifter levels don't coutn towards spell damage)
Wyrmling Breath -- X2_S2_DragBreath.nss
Captivating Song -- X2_S1_HarpyCry.nss
Basilisk Stone Gaze -- X2_S1_PetrGaze.nss
Manticore Spikes -- X2_S1_GWSpikes.nss (This is a Shifter's Main attack form, and it's worthless ATM becuase it shoots 5 spikes at 5d4 damage each.
Illithid -- X2_S2_GWMindBl.nss
Azer -- X2_S2_GwBurn.nss
Death Slaad Lord -- X2_S1_ChaosSpit.nss
Stone Golem -- X2_S1_HurlRock.nss (This hurts the shifter, and allows for evasion reflex save, so the DC should be extremely increased. Also, the damage is very low even at 37 Shifter Levels (Level 54, 10 druid 30 shifter base, 14 LL, so 17 druid 37 shifter)).
Iron Golem -- NW_S1_GolemGas.nss (Poison Gas DC Tops out at 17).
Since you refuse to alter NWN files, and thus allow items' properties to be carried over to shifted shapes, it's only fair you increase these significantly.
Alternatively, you can raise them (regardless, they are in TERRIBLE need of upgrading) and add magical scripted effects onto shifted forms, for instance, when one shifts into Stone Golem, to add 100% slash/pierce immune, 50% bludgeon vulnerability. Anything, defense-wise will benefit a shifter, because when they shift they are naked, only whatever you wear in your chest slot carries over to shifted forms.
Also, for legendary levels, Shifters need an advanced ability-scale, since legendary levels are divided between classes, one cannot get even the 60 (or 50) maximum for the class, but only 40 levels tops: 30 base, 10 legendary.