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Forensic photography is not about small depth of field (which is used in artistic photograpy). You need big depth of field (DOF).
FX vs. DX of same camera generation will give you about same image quality at given DOF . Newer generation cameras compared to older have better per area light gathering effectivity so they can get better image quality at given DOF. FX lenses of same price are sharper in center than DX lenses, but possibly less sharp in corners. VR can help you with static scenes. If scene is lit with flash only, you can use shutter speeds up to flash sync speed without problem (faster speeds in HS mode decrease flash power). If scene requires both ambient and flash light, you need the camera to be stable with speeds like 1/60s or 1/30s - VR can help there. Flash seems to be very important to you - chose one with faster re-charge, not to slow you down.
You can see per area light gathering effectivity in this table, in QE column: http://sensorgen.info/
D7100 is not there, but it will be probably similar to D5200. For example D5200 has about twice as good QE as D300 so it can shoot at twice the ISO with comparable quality.
You did not talk about lenses. Do you need wideangle or not? If not, 18-105vr is great. If yes, 16-85vr is great, but two more milimeters is not much improvement for higher price of 16-85vr. Chaning lenses in your environment will be problem. Get dedicated wideangle them. Nikon 10-24 is ok, 12-24 is worse optically (distortion at 12mm), but it does not extend with zooming. Note that neither lens is dust or water resistant, but given the budget, water resistent/dust lenses are probably out of scope. Normally lenses outlive bodies, but in your environment, it could be different.
I cannot evaluate water resitance of these cameras. I do not have experience with them in such conditions, so taking that aspect aside, I would choose:
D7100, SB910, 18-105vr.