I did a fair bit of digging into this topic a while ago, but I'm already starting to forget some of my findings. While this thread has wandered in focus a bit, it's probably still the best place for me to note my findings before I forget them further.
First, the switch designations of D2F and D2FC, while they seem similar enough think of as being flavours of the same thing, probably warrants a stronger distinction. Yes, D2F is made in Japan, and D2FC is made in China, but there's more to it than that. The D2F switches are general purpose switches, not targetting specific applications, but are spec'd to try to cover as many different common usages as possible.
The D2FC switches on the other hand, seem to be aimed quite specifically for usage as mouse buttons. There's a presentation for the D2FC-F-K(50M) that specifically introduces it as a Gaming Switch for mouse application. One of the slides points out its footprint is the same as prior D2FC switches, indicating they directly replace earlier D2FC switches that are intended for the same applications (mouses) too.
Regarding switch force, the D2F switches are 150g and 75g for the default and -(01)F variant respectively, with of course some tolerance on these values. Again, being aimed at general purpose usage, these forces are just chosen to cover a wide range of applications. The D2FC switches on the other hand, being specifically for mouses, are tailored to 60g. The have a tolerance of +-15g, so the maximum of this range is 75g, which happens to be the typical force for D2F-(01)F switches. This same number showing up for both switches does not mean they have the same force spec - one is typical, the other maximum. The D2FC is definitely supposed to be lighter than the D2F.
Another distinction is that the D2F is a proper SPDT (3 terminal) switch, whereas the D2FC is only meant to be used as an SPST (2 terminal) switch. The third pin on the D2FC is actually labelled as being a dummy terminal in its mechanical diagrams with the note that it should not be connected to any circuit. The D2F has a contact pad on the NC branch, and has a two-sided contact rivet on the common leaf. The D2FC on the other hand only has a one-sided contact on its common leaf, and the dummy terminal does not have a proper contact pad on it - it's only intended as a mechanical stop and not as an electrical terminal.
This difference may eventually be an issue for me, but we'll see. I modified my mouse with a debounce circuit that puts 0V and 5V on the NO and NC contacts. So far, it's worked great, but if the dummy terminal starts to fail in its NC duty, I'll have unreliable button releases. My switches are already worn to where the NO contact was giving me trouble before I added the debounce circuit, so I'm already beyond my switches' normal lifetime; we'll see how much farther beyond that I can get.
The last difference borne by the datasheets is of the contact ratings. The D2F switches are available with different types of contacts, with the default being 125VAC, 3A (or 1A for -F) silver alloy, and the -01 and -01F switches being Gold alloy rated at 30V, 0.1A. There's also the D2F-5 which is a silver alloy option for 250VAC. The different materials are in the contacts, not the spring as described in other posts.
The Gold alloy is more appropriate for use in mouses, but the D2F-01(F) still isn't ideal. The 0.1A is a maximum load rating, but there is also a corresponding minimum load rating of 1% of that or 1mA. That is to say, if you're using the switch for less than 1mA, the contacts will not necessarily perform reliably to the 2 million cycle spec of the switch. The switches rely on having a certain amount of wetting current to clean the contacts as they open and close, and contact resistance can rise over time if this doesn't happen.
Mouse switches typically would be switching much smaller currents than this, and are thus outside what's recommended for D2F contacts. The D2FC contacts on the other hand are rated for 1mA, and presumably have a much smaller minimum current requirement (I would guess 10uA) that a normal input pull-up should provide.
It's arguable how big a deal the minimum current rating is: without the adequate cleaning current, the switch contact resistance will increase, but with such small currents at play, a mouse can also tolerate much more contact resistance than more typical switching applications. The D2F switches are rated for 2M operations, but this tolerance probably allows them to work to many more cycles as a mouse button than that, whereas D2FC (10M, 20M, 50M) ratings probably account for this already, being their intended application. It's possible even that the contacts are actually the same (aside from the absent NC side), and their different ratings are just a matter of having different definitions for the differing applications.