It's not necessary to learn 40 moves of theory if playing the Ruy Lopez or any other opening for that matter. All you need is an understanding of where the pieces should go, some plans for how to improve your position and good tactical awareness. In Dan's case he is 1400 or whatever in live chess rating. Thus he will likely play here vs 1200-1600 players. None of these opponents will know 40 moves of Ruy Lopez theory and blitz them out any more than he will. We're just amateur players enjoying a game online, not top level GM's making opening preparations for the upcoming Wijk aan Zee tournament.
Improvement plan #10
understanding where the pieces go according to the numerous different defences that black chooses which all have a different character

Not sure I understand the opening.
After 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 b5 5. Bb3 Nf6 the continuation 6. Ng5 pretty well forces Black to come up with some sort of Pawn sacrifice like 6. ... d5 7. exd5 Nd4 since 7. ... Nxd5 looks horrible after 8. Qf3 (or even 8. Nxf7 followed by 9. Qf3+).
in the much worse variation after 10. Nc3 it looks like a fried liver variation where book play pcontinues something like . 10... N6b4 11. a3 N4xc3 ch and there is lots of analysis on other lines as well. Unless you are very well prepared both sides can easily slip. As for the computer analysed variation given, for OTB play it might be good to do some home analysis on variations for black that investigate moving Bd6, g6 and O-O also might look at what happens if black puts Bb7 and c6 after preparation. There seem to be a lot of ideas here and not all of the tactical. While black is a pawn down black does seem to have the initiative to decide where the game will go from here.

in the much worse variation after 10. Nc3 it looks like a fried liver variation where book play pcontinues something like . 10... N6b4 11. a3 N4xc3 ch and there is lots of analysis on other lines as well. Unless you are very well prepared both sides can easily slip.
Yes, but that whole line is worthless with the White Bishop already on b3 (preventing Nxc2+). That's the point.
Any game that ends (in checkmate or resignation) before move #60 has errors in it. No surprise there.
If you want to play the Ruy (or whatever classical opening) 40+ moves deep into the theory and analysis that is 400+ years old, that's entirely a personal preference.
But some luminaries have argued that it's best to learn the game in reverse -- i.e. start with the endgame.
So John Nunn, et. al. is the way to go. Great author. Great thinker. Unlike Doveretsky, who is mostly unintelligible, unless you're rated Expert or better.