Is it forbidden to teach knowledge that one has been enjoined to conceal, and is it forbidden to conceal such knowledge from a questioner who would benefit from it, and is there a consensus on this?

It is not permissible to conceal religious knowledge from those who need it. As for worldly knowledge, there is no sin in concealing it, unless it is among the communal obligations (fard al-kifayat) like medicine and engineering. In such cases, it is obligatory to teach it to those who can lift the sin from the community. And it is obligatory to fulfill a promise if you have pledged to someone to conceal worldly knowledge.