What is the ruling on someone who abandons some forbidden acts for a worldly reason, and is that considered disbelief (kufr)? And if so, is the one who does it excused by ignorance?

The statement that whoever performs an act of worship with a primary worldly motive has committed disbelief (kufr) is not absolutely correct. Rather, it is shirk (polytheism), which could be major or minor. Ostentation (riya') is major shirk if the act is performed solely for the purpose of showing off to people without any intention for Allah. It is minor shirk if the doer intended Allah but also liked for people to see him.

Abstaining from forbidden things is beneficial. Whoever quits smoking for health or a worldly benefit is safe from its sin, but he will not be rewarded for it because he did not intend it for the sake of Allah.