Is it permissible to work for Google, knowing that most of its income comes from the forbidden AdSense program, even though the work itself may not concern AdSense but rather other services, and given that the company is prohibited from rejecting ads?

If the field of work in the company is permissible, then there is no harm in working for it. The fact that the company's money is mixed with illicit or doubtful money does not affect this, as it is permissible to deal with someone whose money is mixed. As for AdSense programs, one should refer to the fatwas related to them.