Hi,
I asuume you will be in Y?
I think the transit in HEL will be reasonably straight forward.
Depending on the class of economy ticket bought the avios and tier points can differ slightly between AY and BA. Details can be found here;
https://www.britishairways.com/en-gb/executive-club/collecting-avios/flights
Regards
TBS
PS As to the turbulence it can vary from route to route and from day to day . I found the LAX route not too bad on BA as generally we flew north ober iceland , and the southern tip of greenland , occassionally there was a bit of chop over the Rockies but nothing too bad.