"But why would anyone need or want to make anybody else responsible for fixing their errors?"
But we don't choose to make anyone responsible, it is God and the Divine Spirit that absolve us of our sins and cleanse us and make us pure, if we ask and believe, otherwise we carry the burden of guilt on our consciences throughout our lives.
"Or here's a thing - what about not "sinning" in the first place?"
But we all sin to some degree. We are covetous and envious and jealous and harsh and cruel and nasty at times and we do not love our neighbours as ourselves. None of us are perfect, we all have flaws and we all hurt others emotionally or by betrayal etc etc in some way.
"if you are responsible for your own mistakes you very quickly learn not to make them"
But it is often only your conscience that calls you to account, since you may have done wrong and have managed to escape the law. Your conscience is your inner spirit, and in some people it can lie dormant for years, but it often surfaces and asks questions of someone who has done wrong. You cannot escape your conscience, it goes with you wherever you go and however rich you may become, because it is your inner spirit. You only need to look at the faces of some politicians who are now rich, but whose torment of conscience is visible on their faces. One day you will have to answer to your conscience, you will have to answer to your inner spirit.
The world won't forgive, humans won't forgive you, only God will forgive you if you ask and truly repent in your heart.
"If someone else keeps coming in and wiping your slate clean, there is no incentive to change is there?"
If you truly repent, then you will already have started the process of change, because the Holy spirit will work within you and begin to cleanse you.
"That's why there has to be the threat of eternal damnation in there too."
I don't believe in eternal damnation, I think that divorce from your true self, divorce from your inner spirit and divorce from the knowledeg of God's love is the suffering.
"we could spend our Friday afternoons contemplating nice things such as icecream and wine, instead of ponderng the despotic nature of the Biblical god and the weird moral compass of his followers."
But contemplating things like icecream and wine, though pleasurable, are not the be all and end all of why we are here. Those are pleasures for the self, they feed the ego, but do not feed the soul. We are here to serve others, to love our neighbours, to not be selfish, to serve God. To love others is to love God. That is our higher spiritual calling and it is more important than physical pleasures.