Well then I'd definitely start at 7.5g and work your way up. It's definitely a less addictive option than methadone.
I've never shot heroin but I did shoot a lot of meth and I've known many people who have. The fact that you're asking about ways out is monumental. It's a long hard road but you got this man!
4 times a day. I used to do 2x but found it harder to taper, so I divided the same dose out into 4. Last time I got completely off it I was doing 6 tiny doses a day at the end.
In my experience how long the effects last is affected by how finely ground the Kratom is. I've had very fine powder that gives a more intense but shorter lasting effect, and gritty powder that lasts 8+ hours.