Will doing an interstate transfer while in the process of getting promoted delay my promotion? Anyone in scroll L03-17?

Take the promotion then IST. Talk to both states so they are tracking. The receiving state will need to complete an LOA which includes the rank associated with the position you will be going to. IST's for officers do take a while. Enlisted can take half a day with good communication. Officers can take weeks to months. That depends on how much work the officers are willing to put in to help themselves.

Just split train in the new unit, while the old unit pays you.

I am on LO3-17. I have promotion orders to 1LT from May 2016... Yes I am closing on a year and a half with my fed rec still not going thru yet.