Four important things I discovered immediately: I had fitted the elbow rests too close together for comfort. I had chosen the middle position of three available. The mirror has to be re-adjusted for each handlebar/aero position. Don't ride on aero bars immediately after consuming any food and drink. <burp> Bare metal is cold to the touch even through GripGrab Polar gloves. These bars definitely need tape, or preferably sponge in winter, on the grips for insulation. A bit of padding [tubular foam] would make them even more comfortable. The forward, aero extensions are not of very large diameter. Offering no chance to fit bar-end levers.
I returned safely home after a 26mph freewheel down a gentle slope ending in a brief steering shimmy! Back at the workshop it took only seconds to re-position the elbow pads on the outer fixing holes. This felt instantly more comfortable while just sitting on the trike. Though it didn't feel quite so obviously "quick" as having my elbows much closer together. Fortunately I can still hold the tops comfortably outboard of the arm rests without them getting in the way. An important consideration.
I found that I was perfectly comfortable even with my nose depressed between the aero bars. My back was easily able to cope with the very low position without the least strain. All thanks to the firmly padded arm rests. Other owners have complained that the pads become sweaty in warm weather. I suppose they could be improved with a perforating drill or alternative [breathable] pad material. The sponge is rather firm, black and closed cell. It is not unlike compact camping mattress sponge. Even the arm rests themselves could be replaced with alternatives. They are held by a central bolt through a notched spacer.
I still need a shop window reflection to get a proper idea of what my back is doing when on the aero bars. Even though it feels very low that doesn't mean by back is flat, nor remotely horizontal. I need to see myself sideways on to get a better idea of how aero I am really getting. I have been fooled too often by my previous riding positions to believe I look even remotely like a "proper" time trialists. Nor a <cough> triathlon athlete. Riding on the drops has always felt as if my bum was higher than my head. It wasn't by a full foot and a half! Nearly 3, wind-buffeted miles, by trike. No swimming stage necessary despite the huge puddles!