Ok, but he didn’t have to make Shamrock be an identical twin to Shanks for that to work.
Him being an identical twin lead to a years long twist about Shanks visiting Mariejois, which people argued and debated for years. Again, this isn´t some super well crafted novel, it´s a weekly comic. Oda is not beyond basically repeating characters(literally Ace/Sabo for example) just for a twist/reveal
There’s clearly something important about their mother’s bloodline
Lol, I don´t think this is clear, but even if you grant that, I don´t know why that means we would assume Shamrock>Garling. Garling is still the one who got promoted to the position of the Gorosei, and is arguably more plot relevant. Shamrock is just a mere knight. Arguably, Gunko has looked better then him, so far, since she actually did handle the Elbaf situation, and folded someone like Gaban.