Will a person be held accountable for his speech if it is understood as backbiting or gossip, while his intention was otherwise, and do angels assess his words based on intention or the apparent meaning of the speech?

Prohibited backbiting is when a person mentions their brother with what they dislike. If what you say about them is true, then you have backbitten them; if it is not true, then you have slandered them. Mentioning boredom with giving salaam is not considered backbiting; rather, it is an expression about oneself, not a mention of another person with what they dislike. One should be keen to spread salaam due to its great virtue in Islam, for it is among the best of deeds, leads to love among Muslims, and is a distinguishing شعار (emblem/sign) for them.