I find that the bees like any dahlia where the centre is open. So they would like the white ones in the photo, but it's not clear with the purple ones. Earwigs would like them as well
Clay shrinks and expands a lot so probably not the best stuff to have beneath a patio as this would rise and fall leading to cracks. It would also be hard to compact the hardcore if it was sitting on recently dug clay.
I didn't realise that there was something called a "legal Garder Designer". There must be a governing body that oversees registration so maybe they could help. Just flick thru our local mag and none of the adverts for garden design give an indication that they are trained or legal.
Why not start with looking at the Royal Horticultural Society website as they do plenty of courses and recommend others. www.rhs.org.uk
I keep mine wrapped in fleece in an unheated greenhouse so would expect it to be OK in your summerhouse. I did lose the leaves one year but the plant regrew from the base so probably hardier than we think.
The RHS plant finder lists suppliers
I would leave them as is for now. In spring cut about half of the stems down to ground level and new stems will grow from the base. The ones you leave will shoot from buds along their length
As always there are loads of common names, that's why the latin names are best. Pheasant Berry is another name for this plant.
I think it's Leycesteria Formosa, also known as Himalayan Honeysuckle. It can grow quite large but you can prune it hard.
I would leave them. The leaves rot away very quickly and will feed the plant.