'Easy' daytime option (example for 20/01): 8:58am EuroCity train departure from Venice-St.Lucia, arrival to Milano Centrale at 11:25am, then you wait for regional train from Milano Centrale (12:20) to Tirano (14:50). Full fare for one through ticket Venice-Milan-Tirano cost 39.25 EUR (if you purchase ticket in advance, then you get -30% discount). In Tirano you will have 50 minutes to purchase ticket till Chur/Arosa (fares quoted in above post), then you will need to make couple changes:
All of the trains on the route from Tirano till Arosa wait for each other in case some a late.
'Cheaper' option: combination of regional trains from Venice to Milan and then to Tirano (use of regional train all the way from Venice will save you up to 15 EUR per person), dep. from Venice at 7:41am, 2 hour layover in Verona, arrival to Milan at 14:00, to Tirano at 16:50, again 50 minutes till next train to Switzerland. As result, you will arrive to Arosa 23:58.