It could be, or the boiler has been drained and refilled or skimmed recently and they didn't bother to clean the glass. Dirty glass is usually way, way down the list of all the crap wrong with a majority of steam boiler installations.
600 looks in the ballpark. I doubt the utility conductors passing through the current transformers could handle 800. 600 would mean 200 through each set of SE conductors, which makes sense.
You've got to be a millennial.
Asbestos fibers are not good to breathe. But at the level that you're talking about there is essentially no risk. Just live your life until you get the test results back. If there's asbestos in the sample, just use a regular old paint brush to put regular old paint over the scraped area.
Your phone is a far greater threat to your health than the ceiling is.