Think the main theory is that Suletta herself might be a clone (or cosmetically altered like Elan might've been) of Eri, instead of Eri being her younger self; for example there's mention of an incident 21 years ago-which might be the one in the prologue. So Suletta is either lying about her age, it's a different incident, or it's possible Suletta is not Eri.
Also the Gundam Aerial includes "Eri" in it. Rearranged that you also get Rei, if they're homaging anime's other famous replaceable clone pilot. Also Elan seems to see a vision of Eri when Aerial does that field thing, and it's implied the Gundam has some sort of consciousness. Basically a bit like the Evangelions possessing the souls of Shinji and (half of) Asuka's mothers.
Aerial is also referred to as "Big sister" I think in one thing. Since Eri would be 25 by now it also fits the theory.
Although cloning and genetic engineering have long been part of both Gundam and their alternate universes, the whole soul-bonding I think is a bit connected to (apart from the Newtype 'ghosts'). The manga/video game Blue Destiny and Unicorn's movie sequel Gundam Narrative both feature Newtypes who are 'absorbed' into their Gundams.