Oh sweet. Battle card management is really annoying, not enough inventory space, 400 cards sitting in my mail box and letting anything below 5* run out, the impossibility to sort the mail box to easily find the cards you care about, both of these problems leading to me mostly not bothering to upgrade shit.
They say the new slot is for special battle cards (but you can't use their skill) ? Why even. Just for the stats ? Because special battle card really don't do shit and there's really no point keeping multiple copies of the same battle card - or even to want to put someone else's special card in your slot.
If I struggled to the end of my determination, to the end of my way of life with my followers, if the result is ruin, then this ruin is inevitable. Grieve. Shed tears. But you cannot regret.