Is it safe to completely finish Discerning the Transmundane and Hermaeus Mora's Daedric Quest before the Main Quest is started. I have it finished and still have the Elder Scrolls in my inventory. After finishing the DawnGuard DLC?

Yes, it is completely safe.
For extra safety you could just not pick up the Elder Scroll until youre on the Dawnguard or Main quest for it, as you do not actually need the item in Discerning the Transmundane.

I do it all the time for the Oghma Infinium glitch. It doesn't interfere with the Main Quest. Not sure about Dawnguard though.

Thanks for advice