Ah, thank you for the explanation. I think I understand your predicament.
May I, nonetheless, offer an alternative view?
In the past, there have been occasions on which one forumite has posted things which show that he has found the answer, without stating his solution... At that point, I have left those challenges alone, for fear of being found wanting in my research and deductions. A bit like a junior doctor not wishing to contribute to a bedside discussion of possible diagnoses, I have felt that I am no longer playing the game, but also, without my consent, playing to the gallery.
I (and doubtless others) would be delighted either to offer some help with image posting, because we need new blood, and recently several threads have suffered near-death experiences, or to hear your solutions and stand by to take up offers of open house.
As you and sycamore have doubtless noted, I am now trying to generate my own images rather than relying totally on material on the web, and I'll try to do more of this if more open houses come along.
Thanks for whatever you have done where you worked on the subject of this challenge, by the way, it's a lovely place to have nearby.
Why the engine choice? I guess the usual cocktail of mass, configuration, dimensions, availability, and price, plus or minus a few others...