Also, yes you can jits ive seen it done.
Could you post a demo? If you're the last person alive and the other team has the flag, you shouldn't be able to suicide.
Primed grenades and timed grenades both take the same amount of time to go off, so you'd have to time them right either way. If you prime a grenade and throw it at an entrance, or just throw it at an entrance, it still takes ~ 2 seconds to go off. Either way, you have to anticipate when a player is going to be there. Also, maps like midnight and roundup have multiple paths, so unless you're all lining up like lemmings, one grenade shouldn't be able to take out your entire team.