Any loss means efficiency below 100%. If it takes 1kwh of electricity to compress 10 kWh of hydrogen (assuming 100% conversion back into electricity in the fuel cell, which its not) then your efficiency is 90%.

This is applied to all steps, electrolysis, compression, transport, and fuel cell conversion back to electricity.

Current fuel cells cannot put out enough burst power to acceleration nor can they accept electricity in reverse to generate hydrogen so all current fuel cell vehicles use a traction battery that is charged by the fuel cell, they are basically series hybrids. They also typically have lower acceleration because the small traction battery + fuel cell cannot supply as much power as a much larger full EV battery.

Regenerative braking works for electric cars because it is super easy to convert movement into electric energy. This isn't true for most other sources of energy H2 and petrol included. Reversing combustion is kind of hard and takes up big chunk of the linked graph.

Probably because you'd need a battery to store the energy generated via braking thus eliminating much of the environmental benefit.