What is meant by full size rollaboard?
If u can carry it onto Delta, u can carry onto KE.
KE is quite lenient with carryon bags.

KE gives priority boarding to Elite+ members.
U will board before the economy pax.
Unlike USA airlines, there is always space on board for carryons anyway.

Lounge wise, if u have priority pass, u can also go to several other lounges.
Matina Lounge etc
Yes, on KE prestige, u can go into the KE prestige lounge.

In ICN airport, u can go on a free city tour on a long layover but u have to sign up beforehand.
There is also a cultural center inside the airport which lets u make free korean cultural items which is quite cool.