So, I’ve been using this feature quite a bit recently and can answer a few of the questions.
Uploading a CSV or just the line-by-line copy of the text (which is what I generally do) adds to the check-list, it does not replace it.
Taxa that are already on the check-list remain untouched. You get a warning message saying, “taxa already on check-list”.
Names… this is where it gets to be a bit of a slog. Some names iNat recognizes automatically (and occasionally gets wrong). In a few cases the name will refer to several taxa and iNat will ask for clarification. In many cases iNat does not recognize the name, so you have to click the “lookup” button, and if iNat still does not recognize it you need to click the “search external providers” link. If iNat still does not recognize it you then need to go to an external source manually and search to see if it’s misspelled or is a synonym, in which case you copy the updated name into the field and repeat the “lookup” process. In a number of cases, particularly with plants, the entry is a valid and recognized name that iNat simply does not recognize. In that case you can message curators and ask them to add it to iNat manually.
Copying text in line-by-line you’re limited to 1000 lines. I do not know of there is a line limit for the CSV file.
I do not know if adding species in this manner updates info concerning establishment status as I’ve been adding the species in as copied text, not as a CSV due to the way the species lists I currently have are formatted.
One oddity is that even if your ‘project’ includes ‘places’ (completely enclosing them), often the project and the place will have different checklists and numbers of observed taxa. This is a bit of an annoyance and the only way I’ve found to correct this is to repeat the entire process by downloading the data from the ‘project’ and re-uploading it to the ‘place’, then repeating all the manual fiddly bits, which is a bit tedious at times, especially if you’re running into the thousands of taxa.
Hopefully that provides a bit of clarity.