Is the husband sinful if he dies while compulsively paying off an interest-based loan, and will he be held accountable for this debt if he dies before fully repaying it?

If a disbeliever embraces Islam, Allah forgives all their previous disbelief and sins, such as usury, based on the Almighty's saying: "Except for those who repent, believe and do righteous deeds. For them Allah will replace their evil deeds with good ones."

So, the usury that the husband engaged in before his conversion to Islam is forgiven. As for his continuation in paying usurious interest after embracing Islam, he must cease doing so if possible. If he cannot, then he is not held accountable, because Allah does not burden a soul beyond its capacity. He is only held accountable for the principal debt. If he strives to repay it and dies before doing so, then Allah is All-Forgiving and Merciful.