I agree with you. The power between Kyubi and Hachibi is great, cuz just look how half of Kyubis power was able to match TBB from 5BijuusOkay we are going off topic here there ill make it as quick as possible. The power gap between the bijuu's are great. The power gap between the 8 tails and 9 tails is great. So whilst the amber purifying pot sealed the 8 tails, we dont know whether it can contain the 9 tails.
Similarly whether the death god can seal of the 8 tails is unknown, it just came up against a much greater bijuu who's chakra is the greatest of them all, excluding the 10 tails.
I agree the pot is supposedly one of the sage's weapons, whilst we dont know who the spiritual weapons belonged to. True nobody can for definite say that the totsuka can seal a tailed beast, but similarly we cannot say that the pot can seal the 9 tails even. What im saying is that if one legendary weapon whether the sage's or not can contain a bijuu, i see no reason why another cannot, but nobody knows the answer to this. The closest to this is what the pot has achieved
Ok for the topic, like i always say, no Minato isn't immune to genjutsus so far we know, but that doesn't mean he didn't had his method to fight Uchiwas. He did a war and became a hokage in a period where there were still Uchiwas, and his wife was even closest friend from the wife of Uchiwas leaders(Itachis father). So he must have knew how to fight them and their genjutsus. Unfortunately i've no proof for that.