By now I can respond to my own question, at least partly. XEEST seems to be owned nowadays by Grupo Siete, and they have a website at
http://www.gruposiete.com.mx/ - again it is Flash site without any visible email info, but I will try calling them. You need to click "entretenimiento" and "AM" to get to the listing of their AM stations, of which there are only three. If the info is correct, also 1240 XEWG in Cd. Juárez has changed hands from a local company to this DF-based company. The calls on this site are spelled XEEG, which is probably an error because changes in calls in Mexico seem to be extremely rare.