And is also extremely viscous, so might not flow quickly, which is why I suggested a slowish drainage/refill rate.
Yes, but we're also sort of taking for granted
the most unrealistic thing about magma in this game, which is that it always stays a constant temperature, even if pumped out of the earth for years, and can melt infinite material (as in, transferring its heat energy into that material) without ever dropping in temperature, even if it's a single tile filled with magma.
Keeping current viscosity should be relatively fine.
The thing about a possible eruption mechanic would be that it would probably try to just create, say, 10 tiles, or 70 * 1/7ths of magma starting at a certain point, and look for the path of least resistance from that point to put all that magma.
If there's open air, it would just pump magma up into that. If there's no open air, it would start testing different types of items blocking its path, including solid walls, for resistance values, and try to break down those materials if they have enough pressure (measured in how much magma it has to spew forth) to see if it breaks down a wall or a door or something.
If not, then the magma simply waits until there is a change that will let it break through (like opening that door or mining a tile away) or else will wait with that existing pressure until the pressure rises again before trying to break through doors or walls.
If it does have the pressure, it punches through rock to create small magma pipes to fill, continuing until it is stopped.
If magma recedes, it should leave behind igneous intrusive stone types instead of just empty tiles. There may be a small chance of an igneous intrusive ore or gem.