Minecraft-Java-Edition – How to Configure a Redstone Energy Cell to Emit a Redstone Signal When Full in Minecraft

Use the Redstone Conduits from the same mod, since they're smart enough to stop the Energy Cell if no energy is requested by machines that they're connected to directly. (The recipe involves filling empty conduit with molten redstone in the Liquid Transposer, like the Cell's recipe.)

You'll need a Crescent Hammer to set the Conduit to get the energy from the Cell (turn the connection orange).

Based on the numbers yes the magmatic engines would be more efficient in terms of lava used per amount of digging done. However you wouldn't quite get the full 4MJ/t/engine at your quarry because you'll need some sort of power distribution system (e.g. redstone energy conduits) to connect up all the magmatic engines you'd be using, but the loss would be very minor.

The downside of fully powering a quarry with 100MJ/t is that it will run ridiculously fast and your sorting system will need to have been designed (or hurriedly retrofitted) to handle the huge influx of mined materials without clogging up.