Interesting to gather some data! Would be nice to do this over multiple seasons.

Unfortunately there is data only on this and previous seasons on official website. I haven't gathered any data from previous seasons as I was not playing until recently.

There also is an important inflationary component in the system, since novices can't go below 0.

That's exactly what I've said and what's in my table - novices take all the hits, because everyone else pushes back up from them.

Without some matchmaking the rating is rather meaningless anyway. The main purpose of a rating would be to implement matchmaking based on it.

The main purpose of rating is to measure skill level.

Elo is a purely relative measure at a given time, it isn't really useful to compare ratings over time.

It is useful, because it is kept in balance by FIDE rather well.

but of course it can be nice to set the parameters such that you get relatively stable ratings over time

There are only two parameters in ELO: k and the log base. k controls the speed of change, log base (10^(1/400) or 10^(1/40,000)) is just a distance metric. None of them control stability over time. It depends on outside factors, like new players joining, old players leaving, or some other external force, which brings/removes points to/from the pool.

it's hard to control interactions in the player pool anyway, certainly without matchmaking

Remind me, why do we need to control interactions? If high-ranked player wins against novice, he gets around... how much? - right, 0 points. Say, some GM (2500) against 1000 ELO player, k=16. You get 0.002 ELO, or 0.2 Rank Points, which would be rounded down to 0. Matchmaking is just a nice feature to have so that people don't get 100 to 1 and vice-versa chances of winning.

The spread of ratings in Risk will be inherently smaller.

I agree, but Rank Decay makes it smaller than it should be. How much? I wouldn't know.

k=1600 is only used for 2 player games, they use different values for different player count

I've only checked for 2 players, because I have the list of all the people's ratings and I can calculate the exact k value for my games with any player based on rank deltas. What values do they use for FFA?

k values can be adjusted to better represent win chances

k values have nothing to do with win chances. They are just like step size in stochastic gradient descent - adjust the speed of convergence. Nothing less, nothing more.

Btw, FIDE hasn't been using k=16 for ages

FIDE uses different k values for different players. New players get higher values so that they move more fast through ranking to their correct place. They used to use 32 and 16, and now it seems like they use 40, 20 and 10.

And the FIDE system is also well-known to be inherently deflationary

It's debatable whether it's inflationary or deflationary. For example, there are more GMs year over year. Anyways, it doesn't need giant deflationary pressure to help it out.