'But even if both strings of a pair are tuned exactly the same there is a slight time delay (milliseconds) between their vibrations which gives the 12-string effect.'

Quite so! Technically they are 'slightly out of phase'... that's why it's so easy to spot the difference in sound between a pipe organ and an electronic organ even on a recording. The electronic organ's notes are all perfectly in phase, and sound wrong, somehow!

My old Yamaha FG260 has always been tuned in 'Concert' from new, and I have never had tuning issues with it, capo or not. I use the old Jim Dunlop with the nylon webbing...!

I'm known to change the strings when I arrive at a gig, and can see people smirking because they expect me to have trouble with it. It's my turn to smirk when I start playing, because it's rock solid!