You are right that it will not be 100% to the back wheels, even in lock mode it will only be 50% at the front and 50% to the rear. As regards the nose weight on the tow ball do not exceed Dacias recommended weights for that model or the maximum technically permissible laden mass (MTPLM) with the trailer or caravanDuster I don't have my Duster yet but I understand that the "4wd" mode is an "intelligent 4wd system" that will send drive to the wheels with most traction. It makes sense then that if you are towing a trailer that puts a heavy load on your rear bumper and rear wheels the "system" will send a "percent" of drive to your rear wheels. I don't think it is 100% though. I don't think it would ever be a good idea to load up a front wheel drive car (with added "intelligent 4wd notwithstanding") with a heavy trailer load. I've never tried it but it sounds dicey to me. How about those steep mountain descents where you've shifted to first to hold the load back...
Fully agree with you. I thought I would miss the hi/lo ratio box from my old Land Rover but I have to say whilst that first gear can be annoying on the road, off road it works a treat.(First an aside for the "all wheel drive/four wheel drive definition crowd: "All Wheel Drive" didn't always refer to "full time" four wheel drive systems. International Harvester placed the phrase in chrome letters on it's sixties and later 4x4 pickups, Travelalls and Scouts. So there!)
Back to topic at hand. I have now had my 2015 Duster 4x4 for 1000 kilometers. Just before arriving home I have a long very steep hill with a dirt/chat surface to climb. My Nissan Frontier 4x4 couldn't pull it in first gear high range (in 4x4 of course). Always had to shift to low range to climb it. My Duster pulls it just fine in it's low first gear which goes to show that first gear pretty well eliminates the need for a high/Low transfer case. I say pretty well because we all know it won't cover every situation where you would, in a classic 4x4 use low range. So anyway the first time I tried the hill I turned the dial to "4wd" so as not to take any chances and of course no problem. The next time I turned the dial to "Auto" and again walked right up with no slippage that I could detect. I haven't tried using "2wd" but I'm pretty sure there would be a lot of spinning and I don't feel like chewing up my new Latitude Cross tires. My conclusion is that the "Auto" setting works just like the "4wd" setting so long as all four wheels are touching the ground. I say resounding kudos to Dacia (well and Nissan for the 4x4 system).