Understand you wanting to get heating restored ASAP...but how did the system allow this extreme temp on pex to happen in the first place?
Sure adding black steel pipe allows one to ignore the root cause of the pex failure, but shouldn't the pex failure cause be understood?
For pex to rupture @ 12 psi there had to be some grossly high water temps. and that needs a remedy. And I doubt a thermostatic mixing valve can do it...mix valve needs to have cool enough water from return to dampen the supply. This water temp that ruptured the pex needed "Dumped" to a zone way before it got hot enough to rupture pex.
Did you have an ice in pipe blockage that might of caused this?
Blockage in pipe raises pressure up to match the pressure pop-off valve...30 psi.
We need to find out the cause...pex is a good product when properly installed.
Steel pipe doesn't like ice blockage either.