Will providing line-level signals into a mic input damage the recording device?
Audio line level is 1.23V RMS or 1.74V peak, this should not be dangerous for the mic input.

If you're feeling extra paranoid, you can add a resistor, about 1-10kOhm in series with the signal wire, to protect the input. Then check if your line level clips the DSLR's input amp when its gain is set at minimum. The resistor will ensure that current flowing into the input is low even if the input protection diodes in the input circuit start conducting. After confirming that it works, you can remove the resistor.

I'll bet that if you check in the manual, you will see it is actually a "mic/line" input, because this feature makes sense. The manual should specify a max input level either in volts or in dBu. I mean, it's a pretty common use case, every soundcard does this, so it would be surprising if the DSLR manufacturer didn't implemnt it.