I can't say anything about the rest of your question, but as for undergrowth, it depends on several factors.

How has the woodland been managed? Most woods in Britain were coppiced, and may well have been completely fenced off. In other countries, wood pasture was quite common, where livestock would be allowed to wander amongst the trees and eat the low hanging leaves and whatever was on the floor.

Coppicing is an ancient system of management where you cut a patch of woodland down to a stump, and let it grow back up again. The resulting regrowth would come in the form of many stems, which were used for a multitude of tasks.

In a coppice, the ground flora you'd find would depend on how recently it was last cut. Within the year or two, you'd probably find plenty of grasses and light dependant species. As the canopy grew over, the plants would begin to change. The grasses would disappear, and be replaced first by slightly less light dependant species, like meadowsweet, then supplanted by more shade tolerant species like ferns, dog's mercury and nightshade, though there are some species that don't fit clearly into any one category, like bluebells, which persist in deep shade but love the sunshine too.

After the canopy had closed over, you'd be left with plenty of tree seedlings too, which would compete with each other for light until they got cut down again when the coppiced wood was ready for harvesting.

There may also be mosses and bryophytes, depending on the pH of the soil and amount of light and moisture on the ground.

If it was in a deer park however, it likely would have been grazed to the ground, in which case the forest floor would be completely bare, save for whatever the deer might find unpalatable.

Source: Oliver Rackham, Woodlands.