Rooting Samsung S5 Duos (SM-G900FD)

I have Samsung (SM-G900FD) with Lolipop(5.0) and I want to root it without tripping the knox. I have been searching the internet for past couple of days and I have found a way. It involves, among other steps, downloading and installing the "towelroot" apk, but here is the problem. Towelroot method does not work with Lolipop and I have to downgrade the software to Kitkat(4.4.2), the software version my phone came with.
Once again I have searched thoroughly to find a way to downgrade to Kitkat and I came to the understanding that I need to download official KitKat firmware from sammobile and install it manually, but I don't know which firmware to download from the list that appears against my phone's model.
I have following questions:
1. How will I know which firmware to download from the list.
2. Once I have downgraded and rooted my phone, can I upgrade to android 5.0

Here is my device info,
Model Number: SM-G900FD
Android Version: 5.0
Baseband version: G900FDXXU1BOE1,G900FDXXU1BOE1
Kernel Version: 3.4.0-4516970, dpi@SWDD5815 #1, Tue May 12 13:27:06 KST 2015
Build Number: LRX21T.G900FDXXU1BOE2
SE for Android status: Enforcing, SEPF_SM-G900FD_5.0_0027
United Arab Emirates: TRA ID : 0016333/08, TA : 00000000/00