"Why should Enable Cef if we need to perform BGP multipath , what if it is not enabled and router is working on Fast or process switching, what issue may cause ,"
What might be caused, and likely will, without CEF enabled, is much slower packet routing, especially of the first packet of a flow. It might also increase the work load on the device.
"can we apply BGP Load Balancing per Packet , or per destination , ( inbound and outbound )
only Scenario I am sure about it is load sharing ( per subnet ) "
First, load balancing would be outbound, not inbound. Second, (as already noted by @balaji.bandi), CEF load balancing is per packet, per flow, which often tends to very nicely/well balance egress link loads. However, although in theory, IP doesn't guarantee flow packet sequence, which TCP does, many applications not using TCP are "disturbed" by flow packet re-sequencing, and even TCP, may initially consider packets arriving out of sequence as having been "lost" in transit, which may invoke TCP's flow rate back-off and/or retransmission of "lost" packets. So, avoiding packet by packet LB is generally highly recommended!
