Not really sure what your problem is, more pictures would help.

What envelopes do is create a zone of influence around the bone.  The bone will affect everything within a certain distance of the bone, and it will affect vertices that are closer more than it will affect the ones that are further away.  It's good for creating nice transitions from one bone to another if you've got two bones affecting the same mesh, say if you've got an elbow you can get a nice bend.  If you use one vertex group for each of the bones and the weights are set at 1 for all the vertices in the group you'll get really ugly bends in something like an elbow; here it is fine because everything is separate.  If you use envelopes you do not have to set up vertex groups.

Personally I don't like using envelopes very much, I find they affect parts that I don't want to be affected by the bone and they don't affect parts of what I want to move so I get strange squashing affects.  I may just need to combine them with vertex groups and it would work, but it seems like using vertex groups and vertex paint mode is easier.

You can have multiple bones affecting the same mesh by using multiple vertex groups, and you can get smooth transitions with vertex groups too.  That's what the "weight" thing that I mentioned in the tutorial but told you to ignore does.  Something that has a weight of 1 in a group will be affected 100% by the corresponding bone.  When the bone moves it will move.  Something with a weight of 0 (or something that is not in the group) will not be affected by the bone.  Something with a weight of .5 will move about half as much as the bone does.  Vertices can also belong to two groups and have a weight of 1 in each--this means that its position when bones are posed will be an average of the two.  (Yeah, it can get kinda complicated)  That might be related to the problem you were having with your spider's body.

For single-mesh joints I like to pose bones then select the mesh and go to weight paint mode and modify the weights at the joints until I get a bend that I like.