Is there a reason why ज़मीन पर cannot go first in the sentence?
I thought pharsh was floor and jameen was earth or land or ground
Yes that's how I understand it as well, फ़र्श is floor and ज़मीन is earth/land/ground. Though perhaps colloquially ज़मीन is used for both.
Yes. ज़मीन can be used for both.
theoretically speaking - मेरा भाई is also in the oblique case, am i right?
In this sentence, only ज़मीन is in the oblique case because it is the object of the postposition पर.
If मेरा भाई were in the oblique case, it would become मेरे भाई.