for valve replacement you usually take the one you need to replace out (with the water cut off) and take it down to the hardware store. that is if it needs replacement. sometimes you can just replace the rubber part, rubber O ring or washer if its leaky. thats if its not a mechanical issue with the valve. but i dont sense that that the prob. slow flow is cruddy pipe and failed joint, i think. They might have put the shower in the middle to eliminate splash over ?  but the conventional wisdom is to set the shower up at the head of the tub. take a picture of that part if needed. he would just rough in new copper pipe to the tub faucets and put in a new brass fixture with a shower diverter and run some pipe up to the new shower head. so then there would be three knobs, hot, cold and switch to shower knob in the middle. it would need a different brass thing than you have. the brand i linked to is higher end $$$ but which ever the valve solid brass one he can find is fine. we are not talking too serious a money.