multi fuel stove with back boiler

You need specialist advice about the make/model of stove, but, usually, it is possible to use the stove after the back boiler has been decommissioned. You need to fill the back boiler up with sand, through the pipes, then close them up, but leave a vent on one of the pipes to allow for air expansion in the back boiler. It should be totally dry before attempting it. Fresh dry sand will pour in easily.

Same topic was discussed here a few weeks ago. I agree that you have given the "standard" advice but some of us are not quite sure what benefit the sand actually gives. While it might reduce long term distortion, it seems to me it will increase the temperature differential between front and back by reducing radiation and convection.

The really important point is of course to make sure the boiler is vented, and will remain so for ever. So I would be inclined to leave both pipes open, and perhaps drill a hole or two in the boiler as well. Particularly important if there is any possibility of water ever getting into the boiler (from leaks, condensation, etc).

Actually you just need to make sure that the back boiler isn't sealed up. Been running a stove with a back boiler for 20 years with no water in it