Is it legal to score a goal / points by inserting/pushing the ball into the bottom of a goal, rather than scoring by placing into the top of the goal?
Ball scoring direction
Please review the Q&A Usage Guidelines, specifically point 3, "Quote the applicable rule from the latest version of the manual in your question.".
There is no rule prohibiting this action. Therefore, it is legal, provided that no other rules are violated in the process, such as SG5:
<SG5> Balls may not be de-scored from the top of Goals. Balls that are Scored may not be lifted by any means such that the Ball goes above the top edge of the Goal.
It is expected that while removing Balls from the bottom of the Goal, this may cause the top Ball to momentarily go above the top edge of the Goal. This would not be a violation of this rule and is considered to be normal game play.