Any recommendations on a good shady spot to head for getting a sun lounger when you arrive?
Do you have to get there exactly at park opening to have a chance of getting a lounger? We will be going on a week day mid April overall CL predicted 6. - I think by this point in our trip we will welcome a bit of a lie in 
How long would you allow for bus travel from POR to BB, do we have to go via AK?
I don’t have any experience, but want to know for our trip in Sept. But there are now direct buses from resorts to Water Parks . I did see that some resorts share buses, for example POP and AoA share a bus. I was going to try to get there around 9:30 to rope drop and get locker, chair, etc and to get in line for the chair lift. I figured it would be later than rope dropping. Maybe a bus will come around 9ish and that will get you there in time? Hopefully someone has recent experience with it since the bus change.
Cross the bridge after the entrance, shops and changing area and turn left. There’s a series of sand areas with loungers and cabana shelters that’s a little back from the beach which we really liked. No problems getting loungers when we got there a couple of hours after opening.
We were at Blizzard Beach on a weekday in early May last year- the park was not busy at all compared to the rest of the resort. Even in the middle part of the afternoon there were still lots of empty loungers. We found Typhoon Lagoon to be busier, but it could have just been the dates we traveled!
We have only been there at opening, but I concur with going to the left for shade. I think if you turn right at the bridge you will be more in the sun. And if you have folks like us, we open the park and then leave after lunch - so our chairs in the shade would be available! It is hit or miss and depends on how many chairs you want obviously. But you should be fine.