The 5 volt pin, pin 27 is where you should feed your good 5 volts.
The switch tutorial uses a pulled down input, I think just to keep from confusing noobs.
It will work fine, but more typical is to use a pullup to 5 volts, and wire the switch between the input and ground. This inverts the sense of the switch - pushed will read LOW, and not pushed will read HIGH, but this is easily accounted for in the code.
Some might point out that with that change, you can use pinMode INPUT_PULLUP and save a resistor. I am old fashioned or stubborn and use INPUT and a real resistor, just habit. The value is unimportant and could plausibly be anything from 4K7 to 33K. 10K is fine.
Drawing the relay in its physical box is confusing; take a look at how the original inventor shows it in his second diagram - drawing the relay coil and transistor in one place and the switching action in another makes the switching action instantly clear… don’t make us redraw the relay or memorize how its pins relate to their function.
The 4K7 at the LED might be a little dim, 1K or 470 ohms will make it bright. Try a few different values and see what you like. Just don’t exceed the maximum current for the LED you select. You seem good at googling or however you do you research, so check out how to select the current limiting resistor for an LED.
The 1K base resistor in your relay circuit is also OK but could probably safely be higher, you just need it low enough to turn the transistor on fully. No large deal, but you could learn about how that choice is made if you were curious,
As the "B" loops were part of the footswitch circuit originally, I don't appear to need them anymore, but have kept them at this stage in case the conversion to latching relays requires a reset path.
That makes no sense to me yet. I may be missing something but it seems like you are mixing two separate problems together.
TBH and in case it isn’t obvious, it is the audio switching that is the part I have forgotten the most about. I haven’t taken time to refresh my memory about send and return and loops and dry and is it wet? I will, don’t bother ‘splainin that here.
I haven’t looked at the original code, have you?