Ask Mr. Smarty Plants

Wow – That’s a bunch of requirements, but we can make some reasonable suggestions.  One of my favorite sources is to use the Wildflower Centers selection of Special Collections.  One of these has been specifically built for Pennsylvania.

The power of using these collections is that they can be sorted for several attributes.  Within your requirements probably one of the most common that will also reduce the list is the height.  Selecting 0-3 feet high – there are 40 possibilities left, which is a reasonable set to start reading.  When I reviewed these – several were deleted for having poisonous parts [we can never guarantee them all!] and only a few listed clay soils as an acceptable base.  You are most welcome to sort and read yourself;  Here are the ones that caught my attention as fitting most of your requirements.

Interestingly, there were a few grasses that came through this inspection.
Grasses, most prefer dry sandy soils: 
Bouteloua curtipendula (Sideoats grama) is the only grass that lists “clay soils”,  Koeleria macrantha (Prairie junegrass) and Sporobolus heterolepis (Prairie dropseed) look like they may adapt,  

There were a small number of low herbs and groundcovers that accept clay soils, consider:
Conoclinium coelestinum (Blue mistflower),   Coreopsis lanceolata (Lanceleaf coreopsis),   Gaultheria procumbens (Eastern teaberry),  Lobelia siphilitica (Great blue lobelia),  Phlox divaricata (Wild blue phlox).