Cascade Trailing Blackberry

How can it be both only female and self-fertile ? Is it actually monoecious ?

Most blackberries, and many fruiting plants (like Asian plums), are monoecious. Some plants are parthenocarpic however, like figs and mulberries, and bear fruit without any pollination.

How far apart do I plant the Cascade Trailing blackberries if at the foot of a hoop shaped cattle panel trellis? Should I then direct them just upwards and over, or should they be instead planted & directed to go up & out along a 4' x 16' horizontally placed cattle panel trellis? How long do the vines get? I have only seen them all over the ground. I have ten plants. Thank you for supplying these!

You can. They are more easily trellised on a horizontal wire setup, but a cattle panel or utility panel works fine too. As they are thorny, they should be trellised for ease of access, to avoid discomfort while harvesting.

I have very thin trailing blackberry vines with fine thorns. Some vibes have heavily serrated leaves and some have "regular" blackberry leaves. They bloom but I have never seen fruit. Are they male natives? Are they useful? Do I add females?

There are yes! Those are obviously male vines. Cascade Trailing is a self-fertile cultivar of the native Rubus ursinis.