Some questions for a returning player.

Just have some specific questions in regards to Stat Caps, Allocations and "Break Points". One other question I have before getting into detail is that I am a summoner main and obviously have 30 points in INT, the issue with that is that when broadening my horizons to other roles I'd like to play SCH when we need heals. Unlike WHM who gets passive MND bonuses, SCH gets INT and with 30 INT for my bonus points I feel like I'm double down the hole on my main healing steroid, how much would that +30 MND be hurting me?

I notice that main stats are linear as they do not really have "caps" per se. What I guess I'd like to know is how secondary stats work. I've heard statements varying from "diminishing returns" to "Break points" and Flat caps. So I do get that the main stats just keep going, but my question is I seem to find many people arguing over DET or CRT and I had a friend that mentioned he noticed "break points" in stats. As in you need X amount of stat to get that stats bonus so if you dont have X amount the current amount of the stat you have does not apply, does this actually exist? From what i gather on stat weights is that DET gives you more per stat vs CRT.