Looking_4_Answers,
I think that a good approach to understanding of Net Vs Gross performance is to appreciate that Net performance is used to ensure obstacle clearance, and, once having satisfied obstacle clearance (with very small margins) Gross performance is used for flight planning. For example the Net 3rd segment acceleration altitude clears the obstacle by a mere 35 feet, but the acceleration altitude actually planned and used is factored by the Net Vs Gross difference, e.g. if the Net obstacle 3rd segment altitude was 300 feet, then the Minimum Acceleration Altitude (for a 2 engined aircraft at the minimum 1.6% gradient) would be
(300-35)*(1.6+0.8)/1.6+35 = 432.5 feet, i.e. 167.5 ft obstacle clearance in lieu of 35 feet.
Similarly, if discussing en-route OEI altitude capability, the Net Ceiling is used to comply with obstacle clearance, and, having done so, Gross performance at the higher altitude used for flight planning.
With regard to your 1st segment concerns, true, at first sight the requirement to merely have a positive rate of climb may seem alarming, but this is only a certification requirement. In actual runway analysis, the actual gradient for the weight and environmental conditions is computed against the known obstacles, and obstacle clearance must be ensured. In reality most twin engined aircraft achieve at least 0.8% net in the 1st segment (types WILL differ). If the 1st segment obstacles ARE limiting, then in most such cases the 1st segment limit will govern the Maximum Takeoff Weight. In extreme cases, it may be necessary to 'wear' a large Takeoff distance penalty and accomplish the 1st segment above the runway.
Regards,
Old Smokey