Does this refer to one sin or additional sins? For example if you commit one sin which is normalised, does it mean that your heart is indifferent to other sins?
Referring to the darkening / encrusting of the heart: It’s a good question. Generally speaking, the darker one’s heart is, the easier it is to be heedless and disobedient to Allah azzawajal. So, to answer your question directly: No. It’s not always the case. For example, someone can be normalised to drinking but hates missing prayers. It is mentioned by one of the Salaf that they would consider each sin (minor and major) like a mountain about to collapse on them. Whilst the evil-doers look at sin like a small fly they flap off their faces.