Thanks, everyone for all your helpful advice. After doing more research, and even looking for alternative lodgings, we decided to stay at our airbnb. We love to hike on vacations, so we'll consider this a quintessential Positano experience of traversing hilltop streets and stairs. I looked at google map and I am assuming the white and gray dotted lines are stairs that are short cuts from the zig zagging streets like Viale Pasitea. I read the reviews again on our airbnb website, and now I remember why I chose it - for the beautiful view from our high hilltop balcony overlooking down on to Positano and the sea. The daily exercise will allow us to indulge in pasta and gelato guilt free!
So, for anyone else in similar position as me, here is what I found:
The SITA bus schedule for Praiano is more up to date than what is listed on the Positano website. Positano still shows only until June 16, 2019, while Praiano website has the summer schedule from June 17 - September 8, 2019. During the busy summer months, it looks like the SITA bus does stop at both Sponda and Chiesa Nuova stops in both direction.
Here is the link for SITA bus schedule: Sorrento-Positano-Praiano-Amalfi
Here is the schedule for Mobility Amalfi Coast bus schedule for Praiano and Positano: Mobility Amalfi Coast
This bus just goes from Chiesa Nouva to Praiano Marina and back, including stops at Sponda in both direction.
The 12 euro SITA pass will get us unlimited SITA bus and Mobility Amalfi Coast bus pass for 24 hours. We'll be arriving late afternoon from Sorrento on July 24, so purchasing this when we arrive will get us to our airbnb and take us to Amalfi and Ravello the next day. Then we can purchase another one the 3rd day which will be good through day 4 when we leave Positano to go to Sorrento in the morning. Perhaps the buses will all be too crowded and we'll end up walking most of the time, but I feel better that there are options. One reviewer said they rented a scooter near the airbnb, just to get around Positano, but not sure if that's good advice, given the crowd and the zig zag roads.
We may skip the SITA pass on the 3rd day and go back to Sorrento by high speed ferry instead.
Here is the link for ferry from Positano to Sorrento: Positano-Sorrento Ferry