[Understanding SWR & How Antennas Work]

You can NOT measure antenna SWR at the bottom of the feedline, it MUST be measured at the antenna. 

[Largest GMRS Base Station Antenna Avalible?]

The LARGEST UHF Base antenna available without special order are 16-bay dipoles.  On special order you can get a 32-bay dipole.  The real question is how much wind load your roof mount can handle and how much you are willing to spend.

Larger antennas many not help coverage unless you are located a larger HAAT.  You may also need downtilt in the antenna.

[Programming your own radios, and front panel programming...]

It would be nice if the FCC would allows a database search for Part 95 Certification.  I manually checked a few Motorola Part 90 radios and some also list Part 95 Certification.  From what I could determine from looking at the certification files by the manufacturers, all that is required for certifying a Part 90 radio is simply a statement requesting it.

[Phasing Antennas]

If you are talking about stacking one antenna atop the other to get 3-db of additional gain, it requires a new factory phasing harness.

 

If you plan on mounting them otherwise, you would be better to us a voter and a second receiver for phase diversity on receive and can gain 6-db or more on RX.

 

If you are using a duplexer and the antennas are phased incorrectly (harness and/or spacing); it can create some bad TX patterns including rapid fading for mobiles.

 

One of the advantages of using a voter is you can add a high gain preamp (may require a bandpass filter also) and greatly improve handheld coverage.

 

 

[Message exchange with Amateur Radio Service prohibited?]

The rules of both GMRS and Amateur Radio prohibit communicating outside their respective frequencies except in emergencies.

 

SImply put, an Amateur Station can not listen to a GMRS station, and the GMRS station listen to an Amateur Station for the exchange of messages (traffic) by each transmitting on their respective frequencies; aka cross service communications.

 

It is perfectly legal for a GMRS station to exhange traffic with another GMRS  station, and then if they are also licensed in the Amateur Radio Service to forward that message across Amateur frequencies; also a message from a GMRS operator can be verbally or written passed to an Amateur operator for them to retransmit the message on Amateur frequencies. All assuming the message content does not violate the rules of their respective services.