I know that G/25;d5 qualifies as Regular rating, but does G/25;+5 also qualify?
As long as MM+SS is 30 or more, the event qualifies for regular ratings; G/25;d5 and G/25;+5 both meet that condition.
For OTB events, if MM+SS is between 30 and 65, the event will be dual-rated under both regular and quick.
If G/25;d5 is regular rated, why wouldn’t G/25;inc5 be regular rated? G/25;inc5 will always give the players at least the same amount of overall time for the game than G/25;d5, and G/25;inc5 will give the players more time for the game overall than G/25;d5 if at least one move is made in less than five seconds.
Rule 5C notes that “For the purposes of rating G/minutes and inc/seconds (or d/seconds), add minutes (mm) and seconds (ss) for total playing time for each player. That is, total time equals minutes plus (seconds times 60) or mm+ss; e.g., G/60 d/5 = 60+5 = 65 minutes total playing time for each player.”
…that is to say, delay and increment are interchangeable for determining if a game is Blitz, Quick, Dual, or Regular.
About the set time limit. It’s set to see if you play all three Reg, Quick & Blitz against an opponent & playing the clock as an opponent. The clock would be Blitz while Regular & Quick would be against an opponet.