I have a differing opinion....
As for washing after clay, some do , some dont, I dont but my method differs from others, I use only water while claying. Inexperienced clay users sometimes leave a residue behind, thus promoting a rewash.....
being as your dealing with a dark car, a orbital polisher would be best, but may not be an option.
Doing it by hand can yield great results, if your patient, a full detail, and I use a 5 step method, will take 5-6 hrs.
As for a cleaner doing the same as a clay treatment, I totally disagree, otherwise there wouldnt be clays ! Clays are made for deep cleaning, they go beyond what cleaners can do, they efficiently remove industrial fallout, i.e., brake dust, rail dust, chemicals, water spots ( to a degree).
Its a necessity for a thorough detail.
Im not real familiar with Meg's products, but I believe the 1-2-3 punch would be, there cleaner, then polish, then wax.....
My approach would be.....:
Wash
Clay while wet (after rinse) (or with wash suds, then rinse)
Dry (microfiber or Big Blue absorbant towel)
Paint cleaner (removes prior waxes and promotes better bonding)
Scratch removal only if necessary (basically a rubbing compound)
Polish (removes fine scratches, makes paint glossy, preps for wax)
Glaze( if necessary) ((Fills unremoved scratches)
Wax (seals paint surface) ((Rarely U.V resistant))
My products for your circumstance would be:
Mothers clay
3M SMR
Blackfire polish
Blackfire Polymer sealant
S100 Carnauba