...it's called cheat SED deck by chillin because special effect cards during coliseum is just annoying.
SEC works when you place that card among your 5 cards in deck. If you have a lvl 100 N SEC card among those 5, every card you are using in that 5 get their SED boosted by 1.2 during fights in coliseum.
If you farm rank 1s, it basically means they are strong and are likely to be premium players who have SEC cards (so you'll be facing off against crazy high SED decks and can lose easily)

my understanding regarding past lvl 50 is that because you will have played awhile, all N cards will likely be maxed, so you can just N gacha and trade N wc for R wc etc. to level up your Rs so Rs will level faster.

still, I'm surprised about how much you didn't notice...