If you have the mover already fitted you might try a relatively safe test in a field using two off cuts of builders scaffolding planks inclined at least to your estimated 15%.
First see if it will climb, then if it will hold and finally, if it comes down with good grace. Standing well back. That way it will not go far if it winds back, so that significant bit safer.
If it holds, my biggest worry, you could come down with a better degree of control by in effect halving the slope, "crabbing" by moving just a single wheel at a time, alternating. IMO that's the safer exiting policy, though does not get around, will it hold statically.
Even at the best of times the handbrake only uses one shoe a side in reverse, so is not as effective as forwards, but also the van rolls a bit before that single shoe is activated fully. That means the van will have some inertia building up that acts on it tipping it rearwards as the shoe bites. In all I would find out if the mover technique works safely, before taking on the drive itself.