The bare minimum required for replacing a model seems to be the mod files, the textures and ideally mtg (that helps with cutscenes apparently). Chn, ccl and jex are not necessary.

Actually, in some cases the chn and ccl and jex are best to be null. Like if you replace Chris with Irving, it'll crash if it uses the chn, ccl and jex for Chris. But will work if those files are null.

-Chn seems to control hair dynamics. Maybe other physics on the model too? (controls necklace for Sheva too)

-One of the .ahc in soft\ seems to control what the camera angle is

-soft/*.ahc control the camera angles for the character (first one seems to control all the normal camera angles for walking, using guns, etc)

-soft/pl00.hit defines the hitbox. Replace that with a 0 byte file, and the player will have no hitbox at all.

-sound\bgm has the files which points to generic speech

-sound\se\pl\ - generic sounds like grunts and sighs, also footsteps and sounds of the outfit

-sound\se\wp\ - knife sounds

Placement of weapon on back is not defined on jex, chn, ccl or mtg

Model extensions:
2282360D - .jex
4E2FEF36 - .mtg
58A15856 - .mod
3E363245 - .chn
0026E7FF - .ccl
241F5DEB - .tex

Motion extensions:
76820D81 - .lmt

Detailed info about Wesker->Chris model swap:
Rename pl03action.lmt to pl00action.lmt
Keep all textures non-modified
Rename all files in model\ (except textures) to pl00\model\pl00*
Rename pl15\face to pl00\face
Rename pl15\soft\pl00.hit to pl00\soft\pl00.hit
Rename effect\cef\pl\pl15.cef to pl00.cef
Rename se_pl03.srq to se_pl00.srq
Rename emo_vo_03_w.stq to emo_vo_00_c.stq
Rename all sound\bgm\se\pl\se_pl03* to se_pl00*