yes they will add more in the future, if the game lives that long;
you should have 5 coins left at the end;
i wasnt able to unlock only 1 slot cause i actually had to go twice to the 30 to get other items of the same category to unlock (bad luck) but i have 1 almost at 6 on other category;
besides that locked slot, that i have one lvl1 magatama to put there i have another slot with a lvl1 magatama and another one with a lvl2 magatama, all others at lvl4 magatamas.