Is it permissible to give the expiation for an oath (kaffarat al-yameen) to a coffee shop colleague who is in dire financial straits and owes several expiations? And can it be paid in cash, and how is the amount for multiple expiations determined?

If your colleague is poor, it is permissible to give him the expiation (kaffarah). The expiation is on the one who broke the oath himself, so you are not obligated to pay your wife's expiation unless it is a voluntary act of charity. If an oath is repeated and broken each time, an expiation is due for each oath, unless the sworn-upon matter is a single thing and the repetition of the oath was intended for emphasis, not for establishing multiple oaths, in which case only one expiation is due.