Sorry for not replying earlier :-)
Yes, in terms of absolute impossibility, at current rate (updated after France), Mercs would get the title in Japan or Mexico, as you said.
(BTW, in general terms, the numbers of GPs to get the title with absolute certainty would be: n = GPTG/(1+Gap/(44*GPs)), where GPTG is races to go, Gap is current points gap and GPs is number of races already done)
But in terms of very high probability (>99%, almost certainty) the future is much bleaker: Red Bull is already out of the title and Ferrari can be out in the very next race.
Something very, very fundamental has to change for Ferrari can remount the 140 deficit it currently has. Probably it doesn't have any precedent in F1 past, but I have not looked into it. (In Drivers's Championship we have the case of Raikkonen's title...).