Is it obligatory for me to wipe dust off signs bearing the word "Allah" in the street or inside shops, and does neglecting to do so constitute disbelief that necessitates uttering the Shahada to return to Islam?

Glorifying the شعائر الله (rituals/symbols of Allah) stems from the piety of hearts. What you mentioned is not disbelief (kufr), because disbelief occurs when one intentionally dishonors something that contains the name of Allah. The name of Allah should be glorified within the bounds of শরিয়া (Islamic law) objectives, such as removing hardship and difficulty from people, as Allah Almighty says: "Allah does not intend to make any difficulty for you." And the religion is ease, and one who delves too deeply into it may become exhausted and cease (practicing).