Lots of advice here but I AM a certified muzzleloading instructor who is also an active muzzeloading competitor. So my take-
Capping devices ok especially with those iddy biddy #10&11s. Not so much with larger musket caps. But not necessary. Focus on fundamentals before buying stuff then you'll know if you really do need that gadget.
Decapping- if the gun has been fired, they almost always fall off, that is until you're in a speed shoot and they'll always hang and lacerate your fingers. I keep a small knife on my belt that doubles as a tool to remove stubborn caps and also is my patch knife when shooting PRB.
Decapping unfired- get the hammer to half cock slowly and carefully with the gun pointed in a safe direction. Often the cap will come off using your fingers. If it's a tight fit, gently pry it off, again keeping the gun pointed in a SAFE direction. If the hammer is too close to the nipple at half cock, you'll just have to do it at full cock with your finger off the trigger and the gun pointed in a safe direction.
As for clearing the nipple before reloading, two schools of thought. Both have valid points. One says remove the cap to allow the charge to get into the flash channel. Problem with that is if there is the slightest ember in the breech which might not set off the charge, will most certainly do so during ramming because ramming in this case will force fresh air into the channel and ignite the charge. The other school says leave the cap on as there is no oxygen left in the breech from the previous shot and it will be impossible to set off the charge. Personally, I subscribe to the latter. Supporting evidence, review the loading procedure for muzzeloading artillery and focus on the role of the thumbstall. It was kept on the vent through the entire loading process until ready to "prick and prime" and insert the friction primer. That effectively sealed the channel against fresh oxygen until the cannon was loaded. And yes, as part of my competition shooting, I'm on a cannon crew serving positions 1 or 2 on a rifled howitzer in cannon matches.