P: (Windows) Panel is limited to 1600 items (folders, keywords, etc)

The Keyword List panel on Windows LR does indeed have the same limitation on about 1600 items, at which point it fails ungracefully. The problem does not occur on Mac. As Steve said, the workaround is to introduce hierarchical folders and keywords.

This may be an operating system or programming language limit not something LR is doing different than how it was designed, which is what the term "bug" normally means. Any system, physical, or virtual, is going to have some design limits, and the only question is whether those limits will be encountered when using the system. The elevator in the building where I work has, maybe, a limit of 2000 pounds or is 6x6 feet square or whatever the actual limits are, so if too many people want to use the elevator at once, the limits may be exceeded and the solution is to split the number of people into smaller groups and use multiple elevator trips.

If I'm understanding what you're describing, it seems there is some sort of display-items limit, not an actual folder limit since collapsing a folder will let you see more, further down on the list.

I would suggest a simple reorganization of the physical folder structure, either breaking it up under 2 parent folders, a 2000 and a 3000, then move all the 2xxx folders under the 2000 and the 3xxx folders under the 3000 which should double the capacity but would require having to scroll down through 1000 folders with the other one collapsed, which still might be too many, so perhaps a more useful set of parent folders would be ones that have the year-month followed by the first sequence number by that month, so for your example folder, 3853-20150316, the parent folder that would contain it an all the other folders for March 2015 would be 2015-03-3nnn where 3nnn is not the literal folder name, but the lowest sequence number of the earliest March 2015 folder.

The parent folders would have both the month and first sequence number for the month so it'd be easy to locate the correct parent folder when either the date or sequence number is known. I have chosen to put the date part first instead of having the sequence number first, because the yyyy-mm is more of a regular pattern without any gaps so easier to scan down with your eye, and would also allow non-sequence-number subfolders inside if the organization is ever changed, it is unlikely that dates will never be part of the organization, so I put them first in the folder name structure. One more reason to put the date, first, is if you eventually find that older dates are rarely if ever needed, then another layer of parent folders can be made, but this time just yyyy-firstseq so an entire year's worth of subfolders can be collapsed at once instead of having to collapse individual months to get the list short enough to scroll to the bottom of.

There may be some other partitioning of subfolders in a few parent folders that makes sense depending on the subjects in those photos, but these ideas are what come to mind seeing that the existing folder names contain only a sequence number and date.
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Basically I'm saying LR may not be able to accommodate the existing folder structure so the folder structure may need to be changed. Even if LR can be changed, it would take some months or years, perhaps, so what will you do in the mean time? Folder reorganization seems reasonable as a temporary if not permanent solution.