Any predetermined execution order for the begin play events of default classes?

Why not make the order yourself?

There are many ways to do it. The most simplistic way is, adding delays to each Begin Plays of each BPs.

No Delay = First one to trigger in Game Mode
0.01 Delay = Do something in player BP
0.02 Delay = Do something in player Controller

So it will be guaranteed to happen in order. Surely there are more proffesional ways, but that would be a start I guess. I did that before putting everything into right order so they would work without delays. =)