A390
a) "Unless otherwise specified, he gains the effects of his bloodline powers only while in a bloodrage"

b) Dragon Disciple was written before other classes with bloodlines were considered. The only official "revision" is this FAQ.
Going by the intent of this FAQ, the Spell-Like Form of Dragon ability should have a duration of your total effective Bloodrager levels for Draconic Bloodline (so Bloodrager levels + DD levels).

c) based on the FAQ Dragon Disciple levels will still progress your Bloodrager Bloodlline.
You are mixing abilities here. Bloodrager Form of Dragon does not combine with Dragon Disciple Form of Dragon.
Assuming you have 15 levels total, but less than 10 levels of Dragon Disciple you have 2 options:
1) Bloodrage, and gain Form of Dragon 2 bonuses as per 'normal' Bloodrager
2) Use the Dragon Disciple ability to gain Form of Dragon 1 bonuses (regardless of Bloodrage or not)

Note that if you activate the Dragon Disciple Form of Dragon and ALSO Bloodrage, that the Bloodrage Form of Dragon 2 would effectively overwrite the Form of Dragon 1 bonuses, but when you stopped Bloodraging you would (likely) still have the Form of Dragon 1 bonuses.