2) Disadvantages of using a large upperbound: header of line blocking
problem. For example, if a packet is destinated to a host that is
unreachable or has high loss rate, using a large upperbound will keep
retransmitting the packet to the host and prevents transmission of
subsequent packets.

Disadvantages of using a very small upperbound: A small upperbound
means that local recovery may not succeed. Packets that cannot be recovered
locally have to be retransmitted end-to-end, which is more
expensive. For example, a packet from India to UT is lost at the last
hop from AP in UT to the laptop has to be retransmitted all the way
from India to UT. Moreover, if TCP is used as a transport layer, then
the loss will be visible to TCP and reduces TCP sending rate.

3) No. Then the RTS/CTS collision cost is as high as data collision
cost. Sending RTS/CTS just creates additional overhead without reducing
collision cost.