Wait! Re: "This mode applies different tonal characteristics of the selected speaker model to the system's coaxial driver, with the following adjustable parameters." I'm pretty sure the "system's coaxial driver" refers to the whole coaxial speaker, not just the tweeter, which is referred to as the "high frequency compression driver". I'm pretty sure the tweeter is only on in Flat:FRFR or IR modes.
In the section describing the speaker models, the manual also says:
"HF OFF / Natural – This is the natural sound and feel of the speaker in the Powercab with the high-frequency compression driver disabled and no EQ of any kind applied". This might leave the impression that the HF might be on for other speaker models, but it is not. I just tested my Powercab 212 in Speaker:Normal and Speaker:Cream. Adjusting the Global HF Trim through its full range makes no difference in the tone. The HF compression driver is definitely off.
The same HF Trim sweep in Flat:FRFR makes a huge difference indicating the tweeter is definitely on.
There should be no need to use a tweeter to EQ the Eminence speaker to sound like another guitar speaker as the EQ delta should be fairly small. All the speaker model EQs are applied to Flat:LF Flat, and that, like Flat: LF Raw doesn't use the tweeter.
A guess, but I think Flat:LF Flat just EQs the Eminence speaker in a "standard" way so that Line6 could adjust Flat:LF Flat EQ to use with different speakers (i.e., in Powercab 212) and reuse all the speaker models, since their EQ is all applied to the same normalized Flat:LF Flat.
I also tested and Flat:LF Raw and Speaker:Natural don't sound the same. There is some EQ processing that makes Speaker:Natural sound a bit more scooped and brighter than Flat:LF Raw, and not in a pleasant way. LF Raw sounds better to me.
So I think you've been right all along. And I really appreciate your contribution and insights.