What is the ruling on celebrating the pilgrim upon his return, decorating the house for him, saying "Hajjan Mabrooran" (an accepted Hajj), and holding a special party, and is this considered an innovated bid'ah?

Decorating the house for the pilgrim's arrival, celebrating him, and preparing food for him are permissible actions, as this falls under customs, not acts of worship. The basic rule for customs is permissibility. If the expenses are minor, it is not considered extravagance. If the expenses are significant, it depends on the intention of the doer. It is well-established in the Prophetic Sunnah that the Companions celebrated the arrival of a traveler, just as the Naqi'ah (food prepared for the return of a traveler) is recommended. As for congratulating the pilgrim, it is permissible to say phrases such as "May Allah accept your obedience" or "May your Hajj be accepted and your efforts appreciated." The matter in this regard is broad.