Speaking of the TCE, do changes to the Target Curve get applied post-hoc by Audyssey to the reference curve created by the filters (holding the parameters of change constant otherwise), or so they change the actual parameters estimated by Audyssey for the curve fitting?
If it's the latter, I wonder if multiple changes in the TCE can influence the overall fitted shape of the Target Curve beyond the 1/3 octave handles fit by the user.
Just a hypothesis to test....
The TCE tweaks are incorporated, along with the Audyssey Curve, into the filters generated and uploaded to the unit being calibrated.
You can only load one curve at a time. You can re-edit a curve as many times as you care to, but it still will not accept en edited curve that falls outside of the parameters. (It will not let you place an out-of-bounds handle.) If you use a spreadsheet to edit the native CSV file (created by the curve editor) and enter an invalid handle, the curve will not load.