TBH Shayne you don't need to go through all that messing about with clamps etc if you want to remove the pistons to sort the calliper out. Don't remove the calliper,
take the pads out and press the brake pedal. The pistons will travel out one by one, the least resisted first until they are all up against the disc, no matter how stuck they are this will always work. They are designed to not pop out at this point but are virtually ready to do so.
Remove the calliper and do what you need to do with it.