*IMPORTANT*
I found the culprit, it seems to be the LCM gadget. all the other attachments seems to work fine until i put the LCM attachment on the gun.
MDLCache: weapons\v_m16a3.mdl needs to be recompiled
MDLCache: weapons\w_m16.mdl needs to be recompiled
Giving MonteCristo a cw_kk_ins2_m16a4
MDLCache: weapons\v_hands_vip.mdl needs to be recompiled
MDLCache: weapons\v_m67.mdl needs to be recompiled
MDLCache: weapons\v_marinebayonet.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_standard_m16.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_aimpoint_l.mdl needs to be recompiled
MDLCache: weapons\attachments\v_cw_kk_ins2_cstm_barska_xl.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_eotech_xl.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_m40_l.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_aimp2x_l.mdl needs to be recompiled
MDLCache: weapons\attachments\v_cw_kk_ins2_cstm_anpeq_rail.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_flashlight_rail.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_standard2_m16a4.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_kobra_xl.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_bipod_ar.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_carryhandle_m16.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_elcan.mdl needs to be recompiled
Error! Flag "$normalmapalphaenvmapmask" is multiply defined in material "models/weapons/attachments/cw_kk_ins2_cstm_compm4s/aimpointcompm4s"!
MDLCache: weapons\upgrades\a_laser_rail.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_gl_m203.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_foregrip_sec.mdl needs to be recompiled
Error! Flag "$halflambert" is multiply defined in material "models/weapons/attachments/cw_kk_ins2_cstm_acog/trit"!
MDLCache: weapons\upgrades\a_suppressor_sec.mdl needs to be recompiled
MDLCache: weapons\attachments\v_cw_kk_ins2_cstm_cmore_l.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_standard_m16a4.mdl needs to be recompiled
MDLCache: weapons\upgrades\a_optic_po4x24_l.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_flipup1.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_aimpoint.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_eotech.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_laser_sec.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_scope_m40.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_magaim.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_foregrip_sec1.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_elcan.mdl needs to be recompiled
MDLCache: weapons\attachments\w_cw_kk_ins2_cstm_cmore.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_m203.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_sil_sec1.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_bipod_m40.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_kobra.mdl needs to be recompiled
MDLCache: weapons\upgrades\w_po.mdl needs to be recompiled
MDLCache: weapons\v_hands_ins_h.mdl needs to be recompiled
MDLCache: weapons\shells\556x45.mdl needs to be recompiled

The is the console logs from when i gave myself the M16, i feel like that could be relevant in this situation.

Custom models with custom animations, especially weapon viewmodels, are prone to break stuff. You can try any model with this pack but if it doesnt work immediately, dont expect that to ever change. As for attachment world model positioning problem you mentioned, I didnt manage to recreate it by following your steps. Ive tested it in SP, local MP and being solo on dedicated server. MDLCache errors seem to be safe to ignore.