[jhv48]

Agree with the centerforce comment above. Might be the problem. The cause is a clutch that is not totally disengaging for one reason or another.

Now, what are the causes? I experienced much the same symptoms you are experiencing (grinds in reverse even if you engage a forward gear first, hard to shift into first at a stop. But once driving, everything shifted just fine) and threw a whole lot of time and money at the problem in order to solve it. Eventually I did everything you have done so far. Replaced the slave cylinder, fluid, master cylinder, installed heat shields over the lines, adjusted the clutch many times, moved the clutch rod to both the top and bottom holes on the lever, made a new slave bracket to facilitate a straight push on the clutch fork all to no avail. Next, I pulled the tranny and replaced the throwout bearing and pilot bushing and dialed in the bell housing and trans when reinstalled. Bingo! All problems went away for about 1500 miles, and then they returned. After removing and replacing the trans and throwout bearing and pilot bushing three more times, with the problem disappearing and then returning within 500-1000 miles, I decided to replace the entire clutch mechanism from the pilot bushing to the clutch fork, even though the clutch pressure plate (not a centerforce unit) and disc were pronounced in excellent shape every time it was examined.
Upon examination of the throwout bearing, the inside of the bearing, the part that rides back and forth on the bearing retainer from the transmission, was gouged. WTF? Upon further investigation, we found a slight burr on the bearing retainer that was gouging the throwout bearing causing it to eventually hang up and fail to move freely back and forth when the clutch fork engaged it. We removed the burr and polished the bearing retainer shaft and then put all new components back together. That was over a year ago and 2000 miles and the trans shifts like it was made to. You still need to put the trans in a forward gear prior to engaging reverse, but it used to grind when I did this. Now it just slides right in.

So, the next step is to pull the trans and examine alll the parts. Something in there is causing the disc to lightly drag and not fully release. It only takes a milimeter worth or play to cause the disc to remain in contact with the flywheel and cause your problems.

If your car is new and still being handled by the installer, this should be a no cost fix for you. Mine wasn't. If you do pull the trans, replace everything clutch related (from the pilot bushing to the clutch fork) check the bearing retainer for smoothness and have it dialed in before you put it back together. You don't want to have to do this again. Replacing individual parts might solve the problem, but then you might have to do this again if you chose the wrong part to replace.
Good luck, you'll find the problem.