Is the thief can remove my icloud account in any cases

Hello,

My iPad (WI-FI 3) was stolen, Find My iPhone installed on it, when my iPad was stolen directly I activated the lost mode but

until now after about 2 months, the lost mode is bending, I don't know the serial number and all the left to me, my iCloud account

all day I log in into It and I check the lost mode option, but today I noticed some thing, the last 5 character was appearing in

iCloud account --> setting --> my devices, I can't see them as if my iPad was removed from my iCloud account but still appear in my devices when

I go to Find My iPhone page.

Also today someone told me that the specialists can remove my iCloud account from my device, and they can provide the apple support or iCloud the iPad's SN

and they will help them to reformat it, it's true?

the last question,

The Find My iPhone depends on my iCloud account or on the SN of my device?

thanks.