It is an inherited (at least PRC version is) template, so you can only gain it at level 1 ;p but the abilities will scale with your level (stronger smites, better SR, new spell-like abilities etc). It works in very similar way as described (we don't have some SLA's, sorry).
AFAIK Bralani Eladrin is an outsider, and i think outsiders can't select half-celestial template, but i could be wrong here. You will have both spell resistances applied, but they don't stack - only the higher will be used against spells.
You're welcome!