Electronic – lm358 arranged as a 5 volt Buffer

If you intend to run the LED from the same supply as the logic gate, and that gate can source and sink symmetrically, then it doesn't matter. Even though a logic gate may be rated for the same source and sink current, it is still possible that the low side driver has a little lower resistance. The total Vdd and Vss currents the device is rated for may also not be symmetric, even if individual pins are.

In general, it is more likely that low side drivers have a little less resistance and that the total Vss current rating is a little higher. Also, using the low side driver works with open drain outputs. This is why people are used to wiring LEDs between the output and Vdd. This means you want a inversion in there for a logic high to cause the LED to light. Of course if this is all coming from a microcontroller, which it almost always is these days, then digital signal polarity is irrelevant since it can be handled in the firmware either way.

As long as you are not violating any spec in the datasheet, go ahead and wire up the LED whatever way is most convenient for you. You could let the final board routing decide.

This is more a set of comments than an answer per se, but too long to fit in a comment.

Signal from oscillators: 8Vpp 1~20KHz with an offset of ~10V with a new 9V battery.

So the issue is how to couple this to another stage which can amplify it, but at the same time set an appropriate input voltage DC offset suitable to the next stage.

The obvious solution would be to use any amplifier design with reasonably high input impedance, and AC couple to it via a capacitor, so for example a cap from OSC1 to R8.

"The main problem is, on Q1 base, where the signals meet, there's no signal." Whatever voltage signal is at Q1 base will be quite small because the impedance at Q1 base will be small compared to the 1 Meg input resistors. (Especially for frequencies above the knee of the R5-C7 highpass filter.)

So the voltages at Q1 base may well be only 1/100 or 1/1000 of the signals into R8 and R9. In any case what you are more concerned with is the AC currents through R8 and R9 (and thence into Q1-base).

And probably also of concern is the DC voltage at Q1-base -- is it in a sensible range to bias Q1 to operate in it's active range, say with 3 to 4 V DC at Q1 collector? Since you have a 100k collector resistor on Q1, that suggests you are expecting a DC Ic of around 0.03mA to 0.04mA, and thus a DC voltage of rather precisely 0.03V-0.04V across R5 (and not, for example, 0.08V), but there's nothing to set a suitable voltage on Q1-base to make that happen so far as I can see.

Finally, what is the role of C9, 10nF? In parallel with R11 that appears to create a filter that will attenuate output above 160Hz or so, working to considerably suppress the signals in your range of interest, 1 kHz-20kHz.

It's difficult to say anything about what you wrote after "My mission: be able to make its output signal usable" because you don't show a schematic of what your did and it's hard to guess.

FWIW, if you feed an AC audio signal via a capacitor into a voltage follower (which has a high impedance input, hence shouldn't disrupt the source of the signal), you are going to get an output voltage that follows the input voltage. That's assuming you've set the DC level at the follower input to something reasonable. There's not much that can go wrong there, so we need to see exactly what you did that might have cause this to fail.

Bottom line, it looks like your challenge here may be simply understanding how amplifiers work (either op amps or with discrete transistors) and how to satisfy their input requirements for signal voltage or current, impedance, and DC bias (aka offset). Perhaps reading up on that topic might allow you to navigate more satisfactorily?