Even without the gravel, dryshade under a maple is probably the most difficult planting area. I once got Liripe spicata (not L. muscari) going under a silver maple, but it took several years.

Most mints would wince - they prefer a damper environment and dappled light.

Actually sounds like a good place for 'hard landscaping' (garden whimsies, bold planters. garden art, or some of the surface-rooting ferns. Some are actually quite drought-tolerant although the all-at-once leaf drop might be a bit quelling. But it would be for the mint as well.

If you can leave the leaves at the foot of the tree to rot down over the winter then the soil will start to improve - particularly if you were to put on some gypsum to help break up the clay a little.

I have an area with nearly the same conditions. A piece of Variegated Large Leaf Vinca (the kind you see in hanging baskets) accidentally was tossed in the gravel and rooted. It is so aggressive that it quickly covered the entire area and I have to trim it back at least twice a year. However, I absolutely love it and will be transplanting some to another problem area in my yard.

It's good to be reminded that what I would regard as a weed in need of serious control is a relatively polite plant in more stringent conditions! Vinca! Shudder! But you're right!!! It's both pretty and practical.

You might also consider something like Bergenia crassifolia, takes shade, handles root competition, and doesn't mind heavy clay soils. I would suggest that you amend the individual planting holes, use some time release fertilizer, and give it some supplemental water the first summer to get it established. Western Sword Fern might also work okay here.