Hey, no problem. I'm still a learner as well. it's just that i've successfully rooted my phone with that guide. just offering some help here

1. yes, KL3 is a firmware, successor of KJ2. Something like version 2 and version 3. So far, KL3 is the latest firmware for msian sgs2 phone.
2. err, don't know the technical explanation but something like kernel is the main piece of software to run your phone/firmware. When u root a phone, u will root or modify your kernel, so u can install different OS or ROM or firmware. Like in the first method of rooting, that doomlord's method will root your kernel. But it doesnt work for most latest firmware, like kj3. I was on kl2, and it didn't work on mine, with the first method. So, i have to follow the second method, downloaded a rooted kernel (rooted by someone else), and replaced it with my stock kernel on my phone. After that, my phone will be so called rooted. Then you will have to install certain apps, as described on that manual, like superuser, etc.
3. After rooting your phone, u can choose to use back your existing firmware or you can flash a custom ROM. i use back my existing KL3 firmware. The reason i rooted my phone is to have freedom on accessing my files in my phone. I can backup data for some apps, which non-rooted phone can't do.
4/ yes, u have to download the ROM and flash it into your phone, just like how u will do it with the custom rom. You can either use odin on pc or mobile odin,app on your phone.
my 2 cents

Correct me if i'm wrong

Thanks man. I'll really need to understand the guide you link me first and then I will try it out. I am hoping it will be successful.
If something goes wrong what should I do? A hard factory reset helps? I read something on IMEI back up. Should I do an IMEI backup before rooting the kernel? All other data on the phone should be backed up first before doing any form of flashing right?