what is the reason, possible reasons, of a slippery almost oily mouthfeel. I have noticed in my brothers Belgian Pale Ale that it is not right. Not dump worthy, but strange. We cooled a SG and took a sip, and its still "green" but the texture is weird. It stuck on my tongue for a while and was a weird sort of bitter. Not funky, bacteria-ish, not phenolic/estery (I don't think anyway), Not buttery or cream corn, but strange. I wish I would have pulled this up when I was sipping it. It tasted almost like melted plastic smells, and it really, I mean really lingered on your tongue.
So based on the rest of this thread I'll be really careful with the word astringent. That said, when you describe "oily" and "a weird sort of bitter" together, a few usual suspects come to mind. Also, consider there may be more than one thing going on in this beer. If issues start to overlap describing the flavors becomes much more difficult.
First, if mash pH was too high, then you will have astringency in the wort. As described earlier, this is a bit like sucking on a teabag.
This high pH will then carry over into the boil, which means you'll get higher extraction and tannins from your hops (and what is often described as a "more harsh bitterness"). In a more malt-focused beer this can sometimes confuse my senses as I'm trying to figure out if it's "too bitter" or "not malty enough".
Finally oily makes me suspect diacetyl. In large enough quantities, it will lend this oily/slick mouthfeel. That could also come from some sort of contamination, but given 7 days on the yeast and the fact that you say it still tastes green, the diacetyl seems like a strong possibility. Given all of the other flavors you describe it may be tougher to pick out by taste. Doing the warmer temp diacetyl test always makes it more obvious in the aroma to me.
As for testing pH, I've never had any success trying to correlate post-fermentation pH to any pH issues upstream. The brewing process is pretty robust, and from everything I've read and learned pH problems tend to "self correct" downstream, though the flavor damage that was done upstream continues through to the finished product. With that in mind, I'd suggest going back to the water profile for this beer and try using Bru'n Water or your preferred water calc to predict the mash with your grist and the water treatment that was used. I see no mention of any acid added to the mash, but off the top of my head I'm not sure what an all RO mash pH would look like with that grainbill.
Since he used RO it seems like chlorophenols would be a non-issue, but that's what I would suspect based on the mention of burnt plastic. Did he perhaps make a starter with non-treated water and then dump the whole thing into the fermentor? Chlorophenols are detectable at very low levels, so if you make a starter with untreated water that might be enough to detect it in a beer made with treated water.