Is taking a fixed monthly sum from the profits of timber trading, unaffected by gain or loss, considered usury (riba) or an unlawful form of Mudarabah, and is it forbidden for us to take it?

What your uncle is doing by paying a fixed amount to his partners as profit makes the company usurious and forbidden. This transaction is invalid by consensus among scholars. All profit belongs to the owner of the capital, and the worker is only entitled to a wage equivalent to that of his peers. If you are partners in the capital, the profit is divided according to the share of capital each of you owns. This partnership must be dissolved immediately, and your uncle's consent to the dissolution is not a condition.