1.) I can’t comment on this, as I don’t really use Lightroom all that much anymore, let alone in conjunction with Vuescan. What I do know is that NLP doesn’t work with 4 channel DNG files(IR included in the DNG) which leads me to believe, as you are posting on a NLP forum and hence presumably using this software that you are not actually exporting the IR channel in the DNG but probably “baking” the IR cleaning into the raw data for RGB. There are tools to investigate this, I reccomend you peruse this thread and you should be able to find some info.
2.) If you’re scanning at 3600, your scanner/vuescan almost certainly can also do 1800 as the density settings usually increase/decrease by factors of 2. If I were you I’d set it to 1800 and call it a day. My personal opinion is that persuing “ultimate”/archival quality with an epson flatbed is a bit of a fool’s errand anyway. Don’t get me wrong I use one as well for MF images I wish to post online, share with people etc., usually set around 2000 DPI I believe, but I wouldn’t lose any sleep over whether 1800 DPI setting is leaving a little bit of resolving power unused. I highly doubt that’s the case anyway.