I would suggest you go through the chain of command within your county to address the problem. If it's a county owned system, then they need to address it.
However, what may seem like a cool idea such as making a repeater for a repeater, and simulcast comes into legality of licensing and money.
Simulcast is super expensive...that is if you want it to actually work. A fly by night radio shop can slap up some transmitters that are kind of in phase and it will work but sound like crap, or you can have a radio shop that knows what they are doing and have it sound great.
Narrowbanding does reduce coverage. There are many arguments about how much it reduces coverage but we can play it safe and say...yes you will have less coverage.
I would suggest letting the county deal with it, and go through the proper channels to have your concerns addressed. Not all radio systems are perfect and many times it may appear your E911 director is dropping the ball or the radio shop sucks, when in reality the E911 director asks for funding and is denied by county board members.
I see this all the time, old crotchety county board members calling shots and they think the low band motrac is a fine radio and sees no reason to invest any money into the system *sigh*.
It sounds like to me your county would benefit from a multi site simulcast/voted system.
I build analog simulcast systems, and they are pricey but when done properly...you can't even tell it's simulcasted. I get complaints all the time from people who live outside the intended simulcast coverage area saying it sounds bad. A properly engineered simulcast system will work good in the area it was intended for. out side of that area..such as out of county, then yes it will sound bad the farther you get away from it.
interoperability ??? Ummm...analog FM simplex meets that need, and is cheap and simple