ljh505

I think the treaty with Mexico for coordinated land-mobile UHF comms is for 470-512 MHz; link to current treaty dating to the mid-1990s from the FCC page is given below.
 
https://transition.fcc.gov/ib/sand/agree/files/mex-nb/470-512.pdf
 
The treaty region extends 150 km on either side of the border.  The GMRS repeater pairs are 462 MHz outputs and 467 MHz inputs, below the 470-512 MHz range of the current treaty, so you're probably right a treaty modification would be needed for coordinated cross-border USA<->Mexico GMRS operation.
 
Taking another look though Part 95 radio services, though the FCC removed the 250 km contact limit for 27 MHz CB, the only country listed with permissible cross-border contacts at 27 MHz is Canada's CB service (General Radio Service).
 
https://www.ic.gc.ca/eic/site/smt-gst.nsf/vwapj/ric18-issue4-oct08.pdf/$FILE/ric18-issue4-oct08.pdf
 
The clincher seems to be Part 95.1733(a)(9) which prohibits foreign (non-US) contacts for GMRS:
 
§ 95.1733 Prohibited GMRS uses.(a) In addition to the prohibited uses outlined in § 95.333 of this chapter, GMRS stations must not communicate:
(9) Messages (except emergency messages) to any station in the Amateur Radio Service, to any unauthorized station, or to any foreign station;
 
Anyway, good to know. I live in the southwest (New Mexico), so I guess we can't reply to Mexican operators that pop up on the Southwest linked system until some entity works the cross-border treaty issue.

Found a function on the MXT400 the other day that I didn't see in the manual. You can scan for CTCSS or DCS on a channel if you long hold the CTCSS/DCS button. The codes will start scrolling. If there are active communications on that channel, it will eventually pause on the code. I've use it on the repeater channels to help figure out repeater tones if there's chatter.

Nah, the mount is fine.  See the review of the Browning BR-173-S commercial land-mobile antenna I just posted.  The Browning achieved 1.2:1 SWR at 463 MHz on the same mount.
 
There's a couple of comments in the Midland store reviews of the antenna on Amazon that the antenna comes too short from the factory, so I'm not the only one seeing it resonate above the GMRS band.  Again, the resonant point is mount-specific, but if the antenna came a bit longer from the factory, you could cut it to resonate on the GMRS band for a wider range of mounting configurations.  Maybe the MXTA26 is factory cut to be resonant on the GMRS bands for center-of-roof placement on a Midland NMO mag mount?  That's a pretty limited configuration, though.  Many people will use a NMO antenna on a front fender or rear trunk/hatchback/tire rack mount these days, even though the NMO 3/4 inch hole mount was originally designed for center-of-roof placement.

Part 90 software only difficult to obtain if you're trying to get it both cheap AND legal. If you don't mind spending the money, Kenwood is easy to get from any Dealer, and Motorola isn't always cheap or easy, but if you can spend the time to set up an MOL account (Motorola On Line) it's easy enough to obtain. Otherwise, Ebay has about 100 listings on any given day for copied software burned onto discs - and there are sites out there on the interwebs that offer free downloads - although it's not always current/up to date versions & it's nearly guaranteed to be stolen/unlicensed.

Sadly;
 
It is still narrow band +/- 2.5 kHz in the repeater modes. The FCC ID can be searched for the GRANT and the modulation will be indicated. Anything less than 16K0F3E is narrow band.

Probably worth a try. But, I wouldn’t have much hope. The MXT275 appears to be based on a completely different OEM product and, as of yet anyway, nobody seems to have identified it.