"Judaism has a rule that a man must decide to marry his wife" - more accurately (at least with regard to the Rambam you're quoting), a man has to look at his future wife. The reason is "שמא יראה בה דבר מגונה ותתגנה עליו" (Kiddushin 41a). It's easy enough to tell if she's pretty or not before 20. (Obviously both the husband and wife have to decide to marry, or else the marriage never happened at all)

This question will, of course, be much stronger if you can find and add the source for the assertion in the first paragraph, which is the core rule that you're presenting in tension with the guidance toward marrying at 18.

if a father arranges a shiduch, wouldn't it be disrespectful to him as a parent not to decide to marry the chosen woman?

The Rashba basically writes what you said that punishment only after twenty is due to a lack of reasoning ability: שו"ת הרשב"א ח"ו סי' קע"ט וז"ל: והטעם לפחות מעשרים שאינו דן דיני נפשות אפילו שהביא שתי שערות לפי שאין אדם עומד על שלמות דעתו עד שיהא בן עשרים, ולפיכך אינו נידון בדיני שמים עד שהוא בן עשרים עכ"ל.

Similarly, why learn Talmud before one has good reasoning? And why get married before 40 or 50 when they are even wiser?

It is better to get married 'yung' and 'dum' than clever and wise.