Edster, you seam to be missing this, if you retap a transformer delivering 400amps at 200 what ever volts, to 400 what ever volts, you half your current availability down to 200amps. Yes the wiring if suitably rated for the voltage is fine, but a transformer can only transform so much energy - its KVA rateings, its your choice weather you have that as more voltage or more current, but you don't get both.
IMHO though your missing a far bigger issue, transformers are only so efficient. You don't want to be running everything off transformers, especially if you want to AC your space. The idle heat loss of that much transformation is lost then your paying more to remove that heat.
The magnets should be easy to power with a transformer simply running backwards, its done the world over every day and transformers don't care less which way they have the power pass through them.
The parts washer, me i would just run at the reduced voltage, yeah it half's the heating power, but it may prove to be enough, if its not, step up transformer or some more heating elements of the correct voltage. If the elements are old, its really probably a better - cheaper plan to just replace them with correctly rated voltage wise items. Lots of places make custom elements at a wide range of power ratings and there really very cheap, its common to pay sub £5 a foot of element over here for what ever rating you need.
Equally you don't need anything like the sum of your machines name plate current rateings for incoming supply, get a industrial electrician in to spec things using correct derateings then add some head room for future expansion. If your already running everything a couple of minutes with a current clamp meter on your incoming supplier will give you some real world numbers that are far better to base things of.