The M is highly rated on the Verizon website. A coworker uses one of the Verizon Motorola Droids and likes it a lot. Battery life is slightly better than average. The big thing is to set aside a large block of time for her to learn about the phone and experiment with it a lot using the functions and applications she needs and truly wants to use. Do this several times and she will be as comfortable using the M as she was using the old flip clam-shell or solid bar phones. It takes about 3 months to really get comfortable using or not using all the features, getting a feel for how long a battery charge will last the way she uses the phone, finding a phone case and accessories that work for her, and other stuff.
Sprint not only does not have a robust network, they have a very limited coverage one. Same with T-Mobile. If both of those companies had built geographic coverage similar to AT&T and Verizon, you would have more real competition. A lot of people I know would love to use T-Mobile instead of AT&T or Verizon, but their network's geographic coverage is too small. If T-Mobile builds out significant network geographic coverage, then you can watch both AT&T and Verizon have large percentages of customers switch to T-Mobile.
When Comcast was allowed to purchase NBC you saw an investigation in which anti-competitive issues were rejected as a basis to block the merger. If the federal government allowed that anti-competitive merger, then the ones you are discussing have little chance of being stopped.