For the Saxon Bot’s Raid + Surprise, does getting into a space with a higher difference in Population minus enemy Troops count as a “higher priority”?
Only if the die roll in the 2nd bullet of 8.6.5 scores 1-4.
Also, if the Saxons use their Surprise to target an enemy Stronghold, how could you decide which space to use the Surprise in?
The Stronghold with the fewest units protecting it. If too few Raiders arrive in that space, the surprise is lost; that's why the rule uses the expression "in hopes of Assaulting it" - the Assault is not guaranteed to occur.