From a practical and common sense standpoint, you should be able to use whatever available payload capacity you have left for the trailer tongue, as long as you do not exceed the rear axle rating of the van.
If, after everyone and everything is loaded to travel, you weigh the van and you have 1200lbs of payload capacity before reaching the GVWR or RAWR of the van, you should be able to tow a trailer with up to 1200lbs of tongue weight.
The limiting factor is almost certainly the receiver on the van, which is rated for 870lbs.
The aftermarket hitch rated to 1000lbs will let you tow with up to 1000lbs of tongue weight, AS LONG AS you do not exceed the GVWR or RAWR of the van.
The aftermarket hitch manufacturer would be liable if it caused problems, so they have certainly taken some precautions to ensure there will be no issues.
