Attempts to revive a bumblebee are commendable, but one should keep in mind that they only live for about 2-3 weeks (some that only work in the nesting area can live 6 weeks) at any rate. So, it's probable that any such "poorly looking" bee may well be at the end of its life.
Additionally, mites can be internal as well as external so may not be visible.
Finally, although not normally agressive, bumblebees can and do sting but unlike noneybees they can sting repeatedly without harming themselves.
We grow/tend honeybees here in the U.S., but enjoy seeing the wild bumblebees. (You do know the queen and most of the workers hibernate in winter, right?)