Tbh, this thread almost read as a wind up!
If you know a really good solid fuel heating engineer, then let them sort the pipework design out. I have been doing solid fuel heating and various link ups to oil systems for many years and I know the pipework needs very professionally done.
You mention using an expansion vessel, - how would that help when your uncontrolled boiler puts the heat in the sealed system up to near boiling point? (which is actually lower if on higher pressure)
A heat leak rad won't do much to massive excess heat when circulating pump fails or electric power cut.