Regarding the plan B for the two big stages:
- for stage 18 it seems the plan B would be doing Tonale - Aprica - Mortirolo (unclear which side - a local article even implied the one from Trivigno, never used...), then up the valley until Bormio and the final climb.
- for stage 20 today a local article claimed that the plan B is to have Finestre, and that RCS is already seriously considering it, possibly because it needs some minor roadworks to be passable, so they need to commit asap.
Needless to say, stage 20 has the huge problem of passing through another country, with all the administrative issues that may happen in case of the necessity of clearing snow, AND the fact that any plan B that doesn't include Finestre would look like a medium mountain stage.
For stage 20, at the moment it looks like the weather will be quite cold but not too much snowy. I mean, if it snows during the days before the race, it should still be possible to clean the road properly.
In terms of alternatives, clearly the Finestre is the best one. However, if they were not able to prepare the gravel section of the Finestre properly for the race, and if they were willing to change the location of the stage finish in order to save the last mountain stage of the Giro, they should seriously consider Montoso.
This climb has been done in the 2019 Cuneo-Pinerolo from the Bibiana side, I live exactly at the beginning of the climb on the other side (Bagnolo). There are no problems in terms of altitude (Montoso is 1248m) and there is the possibility to continue the climb after Montoso up until Rucas (1530m).
The road from Bibiana is narrow and winding, so they could climb from that side (like in 2019) and descend from the Bagnolo side, which has a larger road in optimal conditions (new tarmac for the 2019 Giro) but still quite technical.
Yesterday I was riding my bike up to Montoso and I was thinking exactly this, they could make a circuit climbing 3 or 4 times Montoso, with the last climb going up to Rucas for the finish. You would end up with a very, very hard stage (Montoso is very similar to the last 10 km of the Agnello, or to the Passo Giau) finishing with the climb to Rucas which is basically Montoso + 4/5 km with irregular gradient (first a false flat, then 10% section, false flat again, 12% section, little descent, 6-7% section, false flat). The circuit Bibiana - Montoso - Bagnolo - Bibiana means you have 9.6 km of climbing, then 10 km of descent, then 5 km of false flat (mainly uphill), then you start the Montoso climb again.