The kids are moving back and I need to add a shower so we don't have to share ours!

In the bath they will share, there's a tub which already has a hand-held shower which comes directly off the nozzle. That is, you pull up the diverter knob on the nozzle, and water goes to the hand held unit attached by a hose to a fitting just behind the diverter. The fitting is half inch, which is the same as the fitting on the shower unit in our master bath, which works great. The old hand-held unit, however, has very low flow. I assumed it was just plugged, but when I removed the fitting from the faucet nozzle I found an orifice of about an eighth inch in size....thus the slow flow. I'd like to do as little plumbing as possible in this project, so it would sure be nice to simply add one of those hand-held units that attach to an adjustable bar, and feed it from the nozzle. No need to get into the wall and add pipe. Do they still make nozzles like this old one, hopefully with a larger opening for the diverted stream? All the hoses I see seem to have half inch fittings, and it looks to me as if the inside diameter of the hose can't be much over a quarter inch. The one in our master bath connects to it's own fitting in the wall, but is the same size. It gives plenty of flow for a good shower.

Also, while I'm asking, I will need to put something on the walls around the tub. There's tile half way up the wall, but I need to use something above that for water protection. I found some 4x8 fiberglass at Lowes, and I think that will work OK. Can I just use silicone to stick it to the wall board? Is there a plastic cove trim that can be used in the corners?

Chuck