Is it permissible to accept invitations from individuals who are benefiting, or are on the verge of benefiting, or have begun attempts to benefit, knowing that the motive behind the work is seeking the countenance of Allah Almighty, and no financial compensation is requested except what covers expenses?

There is no harm in accepting this hospitality, because in reality, it is in return for a favor. The Prophet, peace and blessings be upon him, said: "Whoever does you a favor, recompense him. If you do not find anything to recompense him with, then supplicate for him until you know that you have recompensed him." And whether you actually benefited from them or not, your effort to deliver their rights to them is a favor.