Hello David
I think most of your questions here have been answered in this post here:
Yes that is indeed the case. I think the most important thing here is that it is best practice to implement SoO regardless of whether or not you “need” it. You would never want to route traffic to the ISP that should be routed internally, so by implementing SoO you are safeguarding this behavior.
Of course, that makes sense, but even having that unpreferred route being received can be problematic. If at some point in the future you change the weight or another configuration to influence traffic differently, you may inadvertently cause a loop or suboptimal routing, requiring you to troubleshoot until you find the culprit. By implementing SoO initially, you safeguard the correct behaivior regardless of what other configs you may apply.
I hope this has been helpful!
Laz