Here’s the schematic of the LMNC buffered multiple:
Out-of-loop resistors have the advantage that they help stabilize the op amp against oscillations due to capacitive loads — e.g. if you use a long cable on the output. With in-loop resistors, stabilization requires a capacitor also. That said, such oscillations may not be a significant problem especially with short cable runs. Notice Sam uses 100R outside the loop; into a 100k load, that’s only a 0.1% voltage drop, corresponding to 1.2 cents detuning per octave which should be negligible in most cases.
Sam uses a 10M resistor to ground on the inputs. I think the idea is that it provides a ground connection when nothing is plugged in on the inputs — otherwise the op amp inputs would be floating. 10M is more than high enough to not detune a signal coming through a 1k output resistor. In fact 1M would again contribute only a 0.1% drop.
You could dispense with the switch if you make use of switched input jacks. IN1 tip to IN2 tip switch. Then it’s 2x4 if something is plugged in on IN2, otherwise it’s 1x8.