1: it's necessary remove the sprocket from the camshaft to take the camshaft off? and the tensioner?
Yes. To get the cam out you remove the tensioner blades and the sprocket bolts, shift the sprocket over to the smaller diameter beside where it sits, and work the chain off the sprocket teeth... then the cam will pull out. It will not lift enough to clear the head otherwise.
2: when i will have to clean the old gasket's debris i have particular steps to follow?
You have to clean it before you reassemble. You can get gasket softening spray stuff, gentle scraping is usually all that's needed. A bronze wire brush (hand or wheel) can be used if you're careful and gentle. I avoid steel brushes because they can scratch the surfaces badly, and bits of steel tend to be embedded in the alloy.
3: when i have to reinstall the head with new gasket (i know i have to wet the new gasket with a film of fresh oil)
The tiny seals for the two oil metering orifices are super important. These engine love to seep oil from the head gasket at the ends because those seals are bastards. If your head gasket is a fraction thicker than Honda's original one, it's very likely you'll have oil seeping out. I don't have any magic to get them to seal, maybe someone else here does?
You can coat the head gasket with a few things: aluminum paint or Hylomar are popular. DO NOT use silicon sealant for any gasket surface that extends into the inside of the engine cases (ie head or case halves = never, points cover = maybe if you really love silicon sealant). The sealant will extrude tiny beads into the inside when clamped down, these will get into the oil where they love to clog vital oil passages.
4: where is the correct sequence of cam chain tensioner reinstallation?
Make sure the rubber blocks are in place at the bottom mounts, or on the blade ends. Install the cam, set the front blade in place with the bottom seated properly, set cam in correct alignment with the crank while turning it back to tension the chain a bit against the front blade (only one tooth setting will be right, it's obvious when it's even one tooth out). Undo the lock screw, push the rear blade down so the adjusting mechanism is at loosest, tighten lock screw. Making sure the bottom is seated properly, install the rear blade and its top mount with rubber block.
Once the cam is definitely set with the line parallel to the head at TDC, use a bit of thread locker on the sprocket bolts and torque to spec: these make a big mess if they loosen out. Some on the rear blade top mount bolt is good as well.
Assembly lube should be used on the cam bearing surfaces.
