Fortunately motors of this era run much slower than today's, so IMO it is unlikely that running unloaded briefly will damage it (and the video seems to bear that out.) I determined that the fundamental frequency (pitch) of the motor in the video on high is about equal to F below middle C, which translates to 175 Hz and change. That * 60 seconds in a minute = 10,500 RPM, pretty sedate by today's standards. I wouldn't dare do this with a modern motor, which runs between 30,000 and 40,000 RPM under load.

Motors like yours are called universal because they will run on AC or DC. On both AC and DC, the motor will turn the same direction regardless of how you connect the power source. To change direction would require you to swap the field coil leads going to the brush holders. Swapping the bail wires will make no difference.

The sparking visible in your still image may not be optimum, but it doesn't look worrisome to me. Big fat yellow or orange sparks or a ring of sparks as mentioned previously would cause me concern. A brush seating stone probably will help. A small amount of sparking is always normal in brushed motors. A thin film from the brushes on the commutator is also normal, and it is what permits the motor to run at the speed it does without quickly wearing out the brushes. The brushes are not pure carbon; they often include graphite, which is very slippery. This thin deposited film lubricates the commutator and reduces friction between it and the brushes, but being made of brush material it's conductive and doesn't interfere with operation.

I hope this helps. Best of luck with your 700! I had one in the mid '90s and it was a real pleasure to  own and use.

Cheers,

Joel