OK, here's the latest. The whole Dronexpert RX100 platform idea has been aborted. Dronexpert are not communicating with Studiosport so I have completely lost faith and confidence in them and their product
Studiosport are similarly disappointed and have sent me a returns label/voucher for free postage of the mounting and FPV kit back to them. They say that they'll give me a full refund and then sling the thing back to Dronexpert in Holland.
So, how am I now going to manage to get high quality stills and also a decent standard of video?
Well, I've discovered that if I set the RX100 to continuous shooting of RAW + large fine JPEG and lock the shutter button down, the camera will take a shot every 3 seconds until the card is full. This means that, with an 8 Gb Sandisk Extreme card, I get 17 minutes of shooting, which is about equal to my flight time
So, I have ordered the Zenmuse H3-3D and a GoPro 3+ BE for video use.
For stills I have designed (on paper) a very lightweight platform (made from Wedi board) which will place the lens of the RX100 just below the GoPro (but without risk of fouling the GoPro or the gimbal). Before take off I'll put the Zenmuse in 'FPV mode' and align the angle of the two cameras. This way, with a bit of practice, I can use the GoPro as a 'viewfinder' for the RX100 as it continues shooting at one shot every three seconds.
I will make my life easier by substituting the GoPro lens for one of these
http://peauproductions.com/store/index.php?main_page=product_info&cPath=124_126_127&products_id=582 . This will give me a horizontal angle of view of 78° compared to the Sony's 65.5° and will be close enough to get decent framing (with practice). It also make my videos more cinematic and less 'GoPro-ish' (I hope!)
Anyway, that's it - the end of my Dronexpert RX100 mount experiment. I hope my experience has been useful to others of like mind.
I'll report on the new arrangement when it's been delivered, set up, and is in use (the Zenmuse H3-3Ds are expected in stock at Studiosport on 24th April)