#28915 is enough for 5.6.0, but I noted some non-blockers which should be fixed early in 5.7.x:

McdStorage could use G_DEFINE_INTERFACE, with a GLib 2.24 dependency.

The function currently called get_value, which I suggested you rename to
dup_value, might make more sense if it returned void and either took a
zero-filled GValue as argument, or took an initialized GValue of type @type as
argument. This would make mcd_account_get_string_val() trivial.

> +        if (message && message[0] != 0)

tp_str_empty()?

Can any more of the McdStorage API become internal? I don't see why an Mcd
embedder should ever be calling mcd_storage_load()?

> @@ -1683,68 +1620,33 @@ mcd_account_manager_write_conf_async (McdAccountManager *account_manager,
...
> +        groups = mcd_storage_dup_accounts (storage, &n_accounts);

groups doesn't seem to be used in this method any more: you call this method
just to get n_accounts, emit a debug message with n_accounts in, then free
groups. I'd drop it and just say "all accounts".