> 1 Where should I make the `make-obsolete-variable` call (in which file)? Given it's defined in callint.c it's not
>  really obvious in which .el it should go.

I'd say in subr.el; there's a bunch of those there already.

> 2 I don't have a "current name". Should I make CURRENT-NAME something like "This variable will be
>  removed in the future"? Should I pass "nil"?

The former, but I think the string should start with a lower-case
letter, as it will be displayed after the standard text saying the
variable is obsolete.  I suggest to experiment with different strings
and find the one which makes the most sense.

> 3 What WHEN release number should I use? 25.2 ?

Yes, 25.2.