We have been checked at security probably 9 out of 10 times we've used rideshares when staying at Riviera and have seen people at the gate being turned away. On our last trip, we saw pop-ups in the app when you try to mobile order breakfast at Primi Piatto that say that mobile ordering is restricted to guests staying at the Riviera Resort. Both of these things lead to me to conclude that they have had problems at Riviera with day guests coming in, eating quick service (making it harder for actual hotel guests to get breakfast) and then taking the skyliner to parks (again, crowding the skyliner and creating a longer line for the hotel guests who need it).
To the OP: this sounds like a stressful start to your birthday. First you have to worry about getting past the security guard. Then you have to worry about there being no walk-up availability at the restaurant (very possible). There are plenty of other restaurants that do not have such a high probability of disappointment. I love Topolinos too, but I think it's time to just acknowledge that a bird in the hand is worth two in the bush and book a nice breakfast elsewhere. You can always set an alert to see if someone drops a reservation, and check again the night before when people are most likely to cancel.