Dean,
your getting into the art, not science. A lot of the standard methodologies come from the analog world and the fact only the largest studios had unlimited reverbs, compressors, etc. This way of working (bus or send, fish or bait) was (and is) a good and standarized way of working.
If you need a headphone mix use a send. That way you can vary the amount of each track sent the headphone(s). Use a second send to set up a differerent headphone mix. You can't really alter the amount sent to a bus w/o using a send pot. The same goes for reverb or any other effect. If you want a "room" reverb to emulate a band in a room use a send. that way you can vary the amount of each track that fires the reverb. Less on drums, less on bass. More on vocals. Even more on backing reverbs to make them seem farther back.
Buses are used to treat instrument groups - usually. Guitars go to the guitar bus. So not only are the guitars individually treated for vol, compression and eq, but also that treatment on the bus, along w/ a little reverb maybe, so they sound more alike and breath the same way w/ the bus compression. You might want to shunt the lead guitar to the master to make it sound different. Vocals, same. Drums, same.
It is just easier to use different routing to accomplish certain goals. As to your specific questions - I usually just put a drum reverb on the bus - no need for a send, unless the kick (usually) causes to much bloom in the reverb and mucks up the other sounds. There are no rules for reverb. a live rock n roll song or cd often calls for the same reverb blush over the entire thing. but even here a send is useful for keeping the bloom out of bass instruments. And the vocal w/ usually have a different reverb treatment, whether it goes to the overall 'verb or not. So yes and no on "one space." How you decide what sounds good is the art part - and how you get your sound. You want the whole thing to cohere, but shaped reverbS can do that, even if you don't have a "room reverb."
The main thing I do w/ reverbs is turn down the wet. Most presets are set at 50/50 - I seldom go over 30% for inline work. If you put a reverb on a send turn it to %100 wet and use the send knob to control how much signal gets wet.
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