Gin-Rummy has many scoring variantions. As Andrew suggested in a comment, Paget is a good source for rules of card games.
Some people play that the bonus for going gin is 25 (rather than 20)
and the bonus for an undercut is 20 (rather than 10). Some play that
the bonus for an undercut, the bonus for going gin, and the box bonus
for each game won are all 25 points.
The score on Wikipedia are 25 points per gin, undercut and box bonus.
Both Zinga's popular app Gin Rummy Plus, and cardgames.io use 25 points bonuses for undercuts, gin and box bonus.
For the third question, the answer is no, that player just get the undercut bonus.