I have an 8-inch reflector telescope and a solar filter that goes on the front aperture to view the Sun (most recently to view the transit of Mercury). If I were to use this telescope during a total solar eclipse, would it be safe to remove the filter during totality? Let's assume I also get the filter back on before totality ends.
Yes, you can remove the solar filter during totality - otherwise you will have no way to see the corona (or anything at all) through the telescope during totality.
The dangerous time is (as you also say yourself) towards the end of totality - and it's easy to get lost in time and the joy of the moment. Set a timer to wake you seconds before end, have others observing with you do the same. You absolutely must put back the filter in time prior to end of eclipse or risk your eye sight looking through. If you (only) use it to take images, it's not as critical as it's not your eye being fried but your camera, but don't go beyond diamond ring without filter and camera attached either.
Thus either case: Put back the filter preferentially before the end of eclipse.