2. if i set the send fader to 0, the signal will be 50% dry 50% wet, is this correct?
So I think you're trying too hard to think about wet/dry mix as some kind of x% wet and y% dry. It doesn't work because wet/dry % is not enough information.
Concrete example. A violin dry signal, fader at 0 dB, with a 0 dB send to reverb FX. Person A could say that's a 50% wet, 50% dry mix, because you're sending the same amount of dry signal into the reverb. But then let's say the reverb FX channel is actually set to -6 dB (50% volume). Then that means the dry channel is at full volume and the reverb signal is at half volume of the reverb signal going to the output. So Person B would call that a 33% wet, 67% dry mix. But really there are many other places that volume can be changed, too. The reverb plugin may have an input gain or an output gain knob. If Person C changes that gain knob inside the reverb plugin, the wet/dry mix has definitely changed, but there wouldn't be an easy way to describe it as "X% wet, X% dry". Even more blatant, what if a different reverb algorithm itself has a different natural level of output? then the real wet/dry mix for Reverb A would be different than for Reverb B, even though they would be labeled with the same % wet/dry?
Bottom line: it's better to think of the wet signal as a separate audio signal that you combine at the end, and you can adjust the send level to change the volume of the wet signal relative to the dry one.
3. by default in my DAW anyway, the send fader is at - 6, I guess it's a basic setting, where the signal is more dry than wet, is this correct?
"more dry than wet" is the same problem as X% wet, Y% dry. It's not really meaningful to compare the wet and dry levels like that. It's much more natural to hear it and say "I wish the wet signal were a bit louder compared to the dry signal". In that case you would increase the amount of send, or adjust the level of the reverb FX channel.
4. when my send fader is at - ∞, the signal is 100% dry (if I'm not mistaken), so the sum of the two signals (dry & wet) is the basic signal volume. If I start to push the send fader to the right to add wet, the total volume of the two signals (dry & wet) will increase, finally in my main output the violin will be louder, is this correct? the more I add wet and the more the violin will be loud compared to the basic signal 100% dry, right?
This is technically true, yes. But often times the volume difference is not that significant, just depends on the reverb and the scenario. I've had some situations where I needed to re-adjust the level after reverb, but other situations it didn't matter. If you're concerned about it, it is totally OK to readjust levels after you have set send levels to the reverb =)
5. To adjust the amount of dry, I use the volume fader of my violin track. To adjust the amount of wet, I adjust with the send fader of my violin. To adjust the wet, I can also use the volume fader of my FX channel reverb, but this will also affect all the sent signals that have been routed to the FX channel. Is it correct?
This is correct if you are using a "pre-fader send". i.e. the send level is determined before considering the volume change caused by the track's fader.
But actually I think "post-fader send" is more common, and I think most people may find it more natural. With post-fader send, the track fader adjusts the volume of the track and all of its send FX together. And you can adjust the relative amount of FX by adjusting send levels.
6. The dry / wet ratio adjusts the depth of the instruments in the space. (not to mention the predelay). The more the sound is wet, the further it is, the more dry the sound, the closer it is. That's why for orchestral music, there is often one reverb FX channel per section. right?
Not quite, but close. It's true, more wet mix can make things sound more distant, but it depends on the reverb algorithm. There is more to the "distance" effect than just wet/dry mix. The way that echoes are distributed will change depending on where the instrument is located - front center of the stage, back, side of the stage, etc. People use different reverbs to represent these differences as well, and people also use different reverbs because generally the sound is different and it's useful to have several of them waiting to be used in a project.