Will the donor be rewarded if he donates to the mosque things that are of no benefit, such as wall decorations, or luxurious tiles beneath the carpet, or building a minaret?

Every good and beneficial deed earns reward for its doer if he intends thereby the Face of Allah, and the ranks of deeds and their virtues vary. The examples mentioned in the question fall under the general principle of revering and purifying mosques, even if there are deeds that are more beneficial. As for the tiles mentioned, if nothing of them is visible and their installation offers no particular advantage, then it is more appropriate to spend the money on what is more beneficial.