Valerie, once more, I don’t hold a food safety certification. I know for sure that the temperature which the egg whites are heated from the hot sugar IS NOT hot enough to kill salmonella, that I know for a fact, you can google it.
Regarding using egg white product, like oology, it will work to make the mousseline. It is suppose to be salmonella free since repeatedly tested at the factory as required by their law. But I don’t think this egg white product is any safer than using your own cracked eggs, because it is just a matter of statistics in the numbers. The egg white product can still carry salmonella, I don’t think they test every single batch of product, plus the product travels so far and it is handled so many times, that it is how usually Miss Salmonella catches the ride. Your own cracked eggs goes thru far less hands/handling, less changes, in the numbers, too.
Mousseline is no longer egg. The whites are no longer whites. They are chemically (heated with sugar) and physically (whipping) something else. Plus the butter emulsification. Sugar is a natural preservative, I am speculating that salmonella will grow in Mousseline; butter has a similar effect.
I know for sure, you are not eating eggs, you are eating Mousseline!
In my heart, I believe that eating shortening is worse for your health than eating raw egg! So please, let’s lobby in favor on keeping egg in buttercream (and butter, too).

