I believe there was a thread directly on point as to IAD-EZE upgrades quite recently. A search will likely bring it back up. To summarize though, and based on my own experience, you're very likely to clear. IAD-EZE-IAD is not an elite-heavy route. I have never not cleared and have in fact gotten several op-ups on this route, too.
As for Y seating, row 16 is definitely best. Personally I prefer 16B or 16F, but I like aisle seats. For a window 16A or 16G would be best. In C most people here like 5C or 5D, though I'm not a big fan as you can't reach the pocket attached to the bulkhead without unbuckling your seat belt and getting up.
Finally, yes, I'm certain UA has an interline agreement with LAN but I've never done it. It'd be a pain to claim your luggage at EZE and re-check it at the LAN counter, so I'd definitely try to do it when you check in for your flight with UA. Let us know how it turns out!