Hi! I will try to help you with this as much as I can!
1. Is it okay to use meringue powder in bath products? I am kind of thinking of yeast infection lol
Yes it is okay. In a "normal" bath, the ingredients do not enter the vagina in the first place, and sitting in a tub for the duration of a bath should not contribute to a yeast infection. Ann-Marie actually addressed this in the comments of that recipe blog, if I remember correctly.
2. How long does one cupcake fizzy last prior to use?
Sorry, can't really say, none have ever lasted around my home long enough to test for an expiration date! LOL Really, I see no reason that they should have a short expiration date at all.
3. Sls can you sub it for something else, I don’t know what that is:/
SLS is short for sodium laurel sulfate and it is what makes the bubbles. I personally do not use, but most commercial products (look at your shampoos and liquid soaps) use it. You can easily purchase it at a lot of the soap-making supply shops. As for what to sub for it, I am sorry, but I can't tell you, as that would be reworking the recipe completely. If you have no objection to the SLS, I would just stick with the SQ's recipe. If you do have an objection, then you will have to experiment.
Good luck!