A "real" mandolin is 13". That is really short and not much room to warp. I bet a steel bar would do especially with a 3/8" fingerboard. My Mid-Missouri/Big Muddy mando does not have an adjustable rod, but there is likely a rod in there.
I think there are kit instructions on the web for building a mandolin from Rockler or Stewmac or somebody. If we can find those, you can see how they put in a rod.
A 15 can easily be tuned GDAE, just choose the strings that match with that string gauge calculator thingy.
I put in G D A E for both 13 and 15", and it gives the same answers:
.030, .020, .014, .09
I don't know if folks use alternate tunings for mando much. Probably not because it is such a pain to tune the dual strings. But I'll likely go lighter on the lowest string, like maybe .028 so I can tune ADAE.
And maybe I'll make another and tune DDAd, like that fiddle tuning. I'm playing a lot of D tunes lately anyhow, so that would be a fun experiment.
I built several 4 string mandos ........ no truss rod needed with the typical cbg neck. I think just using a laminated neck and a fretboard would be enuff even for 8 strings. A cbg mando looks tiny compared to a regular mando and like the other guy said ... a neck that short doesn't have a lot of room for bowing.
My next mando will be 8 strings ... I was asked to do one. And this is how I plan to do it.
The Mandola that's my sig is an 3-string model built on a Cohiba box. The neck is laminated from two pieces of maple with a 1/4" oak piece in the middle, and a 1/4" maple fretboard. No truss rod.
No warpage at all after about a year.
It's a bolt-on neck using a furniture insert and a 1/4" bolt.