Hi - there are lots of ways to remove studs - every mechanic has their own pet way to do it, raw heat is the oldest way to do it but the latest way is heat from an induction coil, you can buy them but I don’t know how much they are but they are very good:
Without heat, the safe way: Get yourself an old washing up bowl or a can big enough to get the studs and head in it, place the manifold into the bowl/can with the broken studs on the bottom then soak the manifold flange and studs literally sat deep in the diesel - then let it sit for a day.
Day two: As you still have the studs sitting outside of the manifold - you need a good pair of mole grips - lock them on each stud as tight as you can then try to tighten up the stud just enough to move the stud a mill or two, if they move then try undoing them a little then tighten them again repeating back and forwards till they start to undo properly, if the won’t undo give the diesel a second go. Then repeat the process until they come out.
If you have tried diesel and lots of heat, you could try freezing sprays but for me the only way is a big hammer and easy outs, if they don’t get the studs out, your only way around broken studs is sadly drilling them out.
Ric