Now an update on the 3 grates that I placed into a lidded, plastic tub with some amonia overnight. Pictured above is the center grate, which really wasn't that bad. It is completely cleaned off. No black charring was really ever on this, we're good to go.
The front part of the right side grate is completely degreased, I scrubbed some more black gunk off, but black, charred on grease remains.
Final thoughts on the amonia fumes technique: definately degreases completely, softened some* of the burnt on grease, which I wish to completely remove.
I'll call this 85% satisfaction rate. Only other thing I can think of to try is placing the 2 side grates in a plastic tub and using oven cleaner on them.