Is it correct to use the word "I believe" (أعتقد) intending doubt and uncertainty, knowing that belief (الاعتقاد) means certainty?

If the connotation of a specific word, by common usage, has come to imply conjecture, then there is no harm in using it with this meaning. Words have linguistic, religious, and conventional connotations, and people's speech is understood according to their custom. This is why oaths are referred back to the custom of those who utter them, if there is no intention, because custom may override the literal meaning in some terms.