Time to accept 3 facts.
(1) Shanks is a Swordsman.
(2) Roger is a Swordsman & Shanks/Roger both use the same style of combat like Yamato/Kaidou.
(3) Mihawk's Swordsmanship > Roger & Shanks.
Mihawk was just not the Strongest Swordsman in Roger's Era, he is in Shanks' era.
Mihawk & Shanks being very close to Roger is a possibility now.