1. Do I need to worry about the modern surround receivers mixing my component outputs once it gets them and before they get to the speakers?
Short answer: Yes, but "worry" might be too strong.
There were some "surround receivers" made in the '90s that literally could not reproduce surround program. The only thing they did with the additional channels is generate reverb. They were made to play stereo CD's but use "surround" as a marketing buzzword.
I don't think that kind of blatant thing is sold anymore but that's what to watch out for. Read the details in the manual before buying and stay out of Worst Purchase in general. (That's where scammy stuff like "sound bars" are sold.)
A real surround receiver will have a 1:1 mode. The 6 channels of audio are directed to the 6 speakers.
They should have some speaker management options in addition to that:
Common alternate speaker arrays and speaker management for them:
Removing the bass content from the 5 main channels and redirecting it to the Lfe along with the original Lfe channel for "small top" bass managed speaker arrays.
Mixing the C channel into the front L,R (centered) for a quad speaker array for 5.1 program.
Mixing the Lfe channel into the front L,R (centered) for a 5.0 (or 4.0) speaker array.
A surround receiver may have some what I call "suck buttons" for playing with awkward sounding reverbs and that kind of thing. Look for the setting for 1:1 or the appropriate speaker management setting for alternate speaker arrays.
2. Do the modern receivers simply assumed the signal has already been manipulated (or not, in my case) and simply amplifies the sound and feeds the speakers?
No. They assume the incoming signal (from 5.1) is 5 full range channels and 1 additional bass range only channel (the .1).
Or for the "suck button" features they may assume a stereo source.
3. +'s/-'s of using vintage quad receivers for my setup vs. a more modern surround system receiver?
Line level is line level.* Six (for 5.1 - or 4 for quad) line level channels sent to 6 amplifiers which power speakers connected to them.
Matters not when any product was made. Shop for highest quality for the dollar. Some older products stick around and are sought after because no one has made a newer one that is better.
The 'vintage' input sources are another matter. Encoded vinyl formats and tape formats. Looking to make this as difficult as possible and want to get into the nuts and bolts of the older tech? Then play with those older formats! Want happiness and light and focus on listening to surround mixes? The modern lossless HD digital formats (eg. 24 bit FLAC file) and the computer are your friends.
4. Is it really as simple as it seems!?
Yep! Send six channels of audio to 6 amp channels connected to 6 speakers respectively.
A comment here though: You know how stereo sounds messed up with one channel wildly at a different volume level than the other? Listened to a Beatles stereo mix in someone's car before with only one channel working? Spend a little time making sure all the channel levels are balanced in surround or a mix could get pretty skewed.
* Yes, line level comes in -10 unbalanced or +4 balanced and we could have an engineer's discussion here! Short answer is line level is line level (vs. phono level which is very low or speaker level which is the huge power output from an amp). Tape, CD, aux, tuner, line in - all line level.