If your EXIF is showing changing aperture and focal length even though nothing was adjusted, that points more strongly to a lens communication fault than to exposure settings alone.

One possible cause on the 70D is Safety Shift, which can alter shutter speed/aperture when the camera thinks the exposure is out of range. With manual flash triggers, the camera may ignore flash output and meter only ambient light. But that would not explain random focal-length reporting.

Since both aperture and focal length data were varying incorrectly, and the issue later worsened until the lens physically failed, the most likely cause is an unstable lens: poor internal electrical contact or a mechanical fault causing the lens to report wrong values to the camera body.

So this is probably not a camera-body malfunction. Check by trying another lens; if the problem disappears, the original lens needs service or replacement. Also inspect/clean the lens and body contacts and verify Safety Shift is disabled if you want fully fixed manual behavior in studio work.