Does a person remain a disbeliever if they utter words of disbelief—from which they have repented and declared the Shahada—due to the tongue being accustomed to them, despite their struggle against themselves to abandon such words?

Accountability for uttering blasphemous words depends on the intention behind the utterance. Whoever intends to utter such words is held accountable, but whoever's tongue slips without intending to utter them is not held accountable.

So, whoever insults the religion of Islam, intending the insult or doing so in jest, is a disbeliever. However, whoever's tongue precedes them with a word of insult unintentionally, does not become a disbeliever, and this falls under the ruling of idle talk (laghw).

As for merely accustomedness of the tongue to utter a forbidden word, it is not an excuse for not being held accountable. A Muslim must guard their tongue from what Allah has forbidden.