Yes solvents will degrade HDPE over time, however, they are usually perfectly fine for short term use. HDPE won't melt away in the matter of a few hours by being exposed to alittle xylene or toluene.

As for that apparatus cyantific lol you could try it I like the idea. I imagine the siphon would be right at the bottom of the bucket so it would probably suck up the basic water layer first and then the NP. In that sense you could probably use a beer bottling siphon pretty effectively too or a gas siphon.