Non-Alcoholic Brewing Safety Question (pH)

I'm trying to brew non-alcoholic (0.5% abv) beer using the BIAB method by using a smaller amount of grain than is typical. I'm happy with the resulting beer except that I believe the final pH is too high and the bottled beer is potentially unsafe. I've tried using acidulated malt but the final pH is still too high, so I'm now planning to use lactic acid to reduce the pH.

I've been unable to find answers to the following questions, if anyone can provide any insight it would be appreciated.

1. What is the maximum safe pH at the start of fermentation?

2. What is the maximum safe pH after 3 days of fermentation?

3. What is the maximum safe pH at bottling time?

4. Can lactic acid be added to a ~5.5 pH fermented beer when bottling, or is it too late at that point?

Essentially my question is: at what point in the fermentation process does the pH have to get below 4.6 to avoid the risk of dangerous bacteria/toxins?

Thanks for any help.