Re: low flow from shower
With an actual valve such as you describe, you should have little to no water exiting the tub spout when the shower is engaged. Sounds like it's time to replace the seat washer on the diverter valve stem.
Remove the handle and escutcheon, you should see the stem with a nut around it's base. Use a box end wrench to remove this nut, DO NOT attempt to use pliers of any kind or a crescent wrench, you'll likely distort or crush the stem nut if you do. With the valve stem removed, there should be a washer on the tip held in place by a screw. remove the screw and take the washer with you to the hardware store to match up to a replacement.
It may be possible that the diverter is a cartridge style valve, if that is the case, then you'll need the faucet brand name to be able to find a suitable replacement or rebuild kit.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!