As noted, you can take the bus to DHS and then walk, or boat, over to YC/BC for your ADR. But...that walk will take close to 20 mins...as will the boat ride (not including any wait times for the boat to arrive). This means that for an 8:40 ADR, you would have to leave your resort by 7:30-7:45.
A cab is going to be a much better bet. The cost will be around $10 (each way, not including tip). And it should be pretty quick.
As far as return to POR for departure...what time is your pickup scheduled for?? If you get seated at 8:40-8:45, it will take close to an hour to eat and see all the characters (this is one of our favorite character meals by the way). Then, if you use Disney transport back to POR, plan on it taking up to an hour. So, you finish eating around 9:30-9:45. You get back to POR by 10:30-10:45. You will need to collect any bags left with Luggage Assistence...plan on about 15-20 mins....sometimes it can be very quick, but it can be a longish wait depending on how quickly they work.
You need to be at the DME bus stop at least 15 mins before your scheduled pickup time. The time you are given is not the time to be walking up to the stop...that would be the time the bus is due to depart the resort!!! So, be there about 15 mins beforehand, bags in hand, all ready to board the bus.
