I am in the same situation as you. I have a Platinum Charge and Velocity Platinum, I applied for Explorer and had it automatically linked to the Ascent Premium. Spend on Explorer is earning at 2ppd into Ascent Premium. No sign up bonus was posted after passing $1500 spend. My reason for signing up to this card was an attempt at getting the signup bonus. I called up to "check" that my not having a Gateway MR account was correct, they said yes this is a "benefit" for you, but they didn't offer to unlink and create one, and I didn't think to ask at the time.
Re: your question of rolling the dice and unlinking. To my recollection, in this thread, there have been no examples of users who had a Gateway account created and *didn't* get the bonus. So I feel somewhat confident that if we unlinked from Ascent Premium and had a Gateway account created, the system would give us the bonus all the same. It is a risk, but I am considering going ahead with it. The extra 0.5ppd on earn isn't a big deal to me - I'll be putting no more than $40k/yr on this card, and I'm not thinking long term enough to realistically expect 2ppd with no bonus to be a better long term outcome than 1.5ppd with a bonus.