Easiest to see options with a map - I like this online map - https://tinyurl.com/zekuft9 because of color & big highways vs small. NOTE: cllck & it gets huuuge (on computer not phone); navigate w side-slides.
(1) NAFPLIO - DELPHI) You've identified the options .. neither road is shorter -- and that road along top of Pelops (I've done it by bus) is boring tho easy... So, in the Corinth area, you must make your choice.
However, visiting Nafplio, don't overlook the "NAFPLIO Loop" Driving option to avoid backtracking, see more sites:
• Outbound crossing Corinth Canal, you can't See canal from high-up "Big Green Road"; catch it inbound. About 20km further, exit Big Green at ANCIENT NEMEA... overlooked Gem! It's a "Sacred Games Runner-up" to Olympia - temple ruins, FAB stadium beautifully excavated, NO bus tours! http://nemeangames.org/ - u can enjoy in an hour. Then cross "Big Green" to red road S. to NAFPLIO. During stay there u can visit MYCENAE & TIRYNS nearby, but Save EPIDAURUS for Departure AM ... then drive across PEloponnese "thumb" to ANCIENT EPIDAURUS... then, instead of backtracking to NAfplio -- continue East to sea & N on Shore route to ISTHMIA. Stop to see/photo Canal, & Grab snack/lunch, continue on Big Green to Delphi (I favor the backtrack to ELEFSINA where you head north to Thiva (Thebes) then N West to Delphi... arriving afternoon after crowds have gone.
(2) RETURN to AIRPORT -- you are correct, coming back from DELPHI, you have only ONE key turn... and that is LEFT at ELEFSINA! As this map shows it is the Bypass north of Athens, straight to airport (BTW, if you have to return car w full tank of gas, just 10 km before airport, there's a station right by hiway; saves you $$). The incorrect turn at ELEFSINA puts you into the trafficky-truck-packed route into central Athens, hard to exit from
PS: Altho some maps show highway numbers - NO Greek signs show numbers, just destination names -- (And not repeatedly as USA signs do for us ADHD Drivers ha ha).. a first sign is in Greek, then 500 yard on, 2nd sign in English and then BOOM! the exit! They expect you to stay alert, so driver partner's job is to be eagle-eyed for signs.
