On the black plastic thing you have two easy options.
Take Robert’s approach and just bust it out. It has an outer ring that will break off smoothly and allow enough room for the rocker shafts to come out. I’ve never dyno tested with and without, but don’t believe you’ll see any performance impacts. I think it’s really just a dust and water shield.
Remove the cam gear bolt, remove the black plastic piece, install the cam gear bolt without the plastic piece and do your work. Replace the plastic piece as you complete the job and torque the bolt to 55 ftlbs (I think). The cam gear will not come accidently come off with removal of the bolt, but you should at least hand tighten the bolt before turning the engine just to be safe.
I have never seen OEM or Febi rocker arms fail in that fashion. Like Scott said,rocker arms normally break at the fork on the eccentric side. Most failures are a result of over revving the engine or weak valve springs. Either condition can create a situation where the rocker arm doesn’t follow the cam as it should, the valve to stay open too long, the valve to impact a piston then impact the rocker arm, and cause a failure.
The rocker arm pad failing makes me suspect you have an oiling issue in your cylinder head. After you replace the rocker arms, and before you install the valve cover, start your engine and make sure there is oil dripping from the spray bar at low engine speeds. Pay attention to orientation during the installation of the rocker shafts too; the oiling holes must be lined up.
Is there any visible cam wear on the cylinders that failed? Cams that show any wear on the lobe tips need to be replaced.
For good measure, you should check all over your valve springs and pay particular attention to the cylinders that fail. Check visually for corrosion, pitting, cracking or fracture; any with visible issues should be replaced. Also check for weak springs. This is easy to do with a large screw driver by just pressing down on spring hat. With the valve fully seated (closed), it should take 68lbs of pressure to crack the valve open. If you have a weak spring, you’ll be able to feel it in comparison to the others.
Like Scott said, I would not pull the head unless you need to replace springs or you have low compression on one or more cylinders.