Again my experience is mostly with 67's so I will speak from that perspective. The exterior decor group AND interior decor group (aka "deluxe interior") AND the GT equipment group were all distinct options. You could have ordered none, any one (without the other), any pair, or all three. I have seen pleny of 67 GT/GTA cars with the standard interior and exterior. A common misnomer is the vented hood sometimes referred to as the "GT" hood. Other misnomers are the "GT" wood steering wheel and "GT" styled steel wheels. The vented hood for 67 was part of the exterior decor group (or other specials offerings). The styled steel wheels and wood steering wheel were separate options.
The bottom line is that the following type of Mustang exists somewhere: Hardtop, GT equipment group, 289-2V, 3 speed manual, standard (non-decor) interior and exterior, wheel covers. The GT equipment group added (1) GT badging on fender, (2) side stripes, (3) GT badging on gas cap (standard twist, exterior decor pop-open), (4) dual exhaust with chrome tips (289 HP or 390 only), (5) power front disc brakes, (6) upgraded handling package (but short of the separate competition handling package), (7) fog lamps, ( 8 ) F70x14 tires.
However, if someone had money to order the GT option (along with the minimum requirement of ANY V-8) they would most likely order other options. But the point is they were not part of the GT equipment group option.
For 68 I'm aware of GT equipment group changes in the power front disc brake inclusion, side strips changing to C stripes, GT hub caps on argent or chrome wheels, and possibly not being available with a 2V V8. There were also changes to the hood for all models (not just the GT), making the vented hood more prevalent (possibly standard for a while?). But I don't think that there was any significant change to the GT (such as including the interior or exterior decor group with all GT's).
I wish I could be more help with your particular question on the mirror. I've got a 67 GTA fastback, purchased unaltered from the original owner in 1980. Interior decor, but no exterior decor. Driver's side has remote mirror. No mirror at all on passenger side.
John