Put an accel high voltage coil in 95 YJ, should I open gap-spark plugs?

NO. The Accel coil needs the correct factory plug gap setting to work properly. If you open up the gap, you will induce multiple cylinder misfires.

Is it possible to advance the timing a little to take advantage of the hotter coil?

I'm not familiar with your vehicle, but in some instances a driver can get a different timing curve in another ignition chip.

You might check it out. A few degrees advance should be possible with that coil and you gas mileage will show an improvement immediately, along with a better power curve.