I don't see how that really makes a difference unless there are PCs that are not surprised. Jeral can't cast Shield in the above example because he will be surprised in that first round so he can't take Reactions. Assuming all of the PCs are surprised, all the ambushers can make their attacks, then roll initiative, then proceed. I don't see it playing out differently than rolling init, then rolling surprisers attacks, then proceeding to the next turn.What if Jeral could cast Shield, in your example? I mean, rolling initiative first gives the possibility to use reactions after the turn has passed if the character "acts first". Just asking if it ever came up and how did/would you solve it. I mean, you still roll initiative (possibly starting long before the actual situation happens - using a set of pre-rolled results, prehaps, to mantain the tension) but how would you convey such a possibility?
I've tried to solve it, and came to a " you hear the sound of a crossbow snapping" as the point where the situation halts and initiative is rolled. If the party is already suprised - and that has been determined beforehand - i've found that this solves the narration problem and frees me of 5 rolls used, letting the players still have a bit of tension and the moment of the "huzzah!" if rolling high, because they know they might be able to do something useful to help anyway.
Not that full blown ambushes are that common nowadays in my game, sadly.
Edit: Before any possible post, possibly: Yeah, i've been stumped at seeing that shield does not require seeing the attack to be able to protect oneself. Nor do reaction require to see the source. It is... eeeh.
Yeah that is annoying, but they still need to have had a turn.