Root Samsung Galaxy S5 I have rooted my Samsung Galaxy S5 using Odin to first download (Philz Touch) Recovery, then downloaded Custom Rom 'Omega'.

My first question is if I unrooted it, would I get my Warranty back? 
Second, how can I unroot it?

Third, before I rooted my phone it used to lag a lot and the battery used to drain fast. When I used to open my battery settings, it showed me that (Android System) was draining my battery. I did factory format, and still the same thing. I did everything from Force close unwanted apps to turning off unneeded options (GPS, Mobile Data, Bluetooth) and it did not improve. Then I decided to root my phone, and nothing improved. What should I do?

My Phone is SM-G900F, 4.4.2 Kitkat, bought it from overseas and now using it in the US.
No. A warranty is a legal agreement between you and the manufacturer. You can't break a contract and expect to "undo" it as if it were a typing mistake in an email.