You are correct that the main difference between the games was the number of formations in 'the front line', or within 15cm of the enemy. In the 1st game Vs the Eldar, the Elysians had 3x Drop infantry and 9x sentinel squadrons in 15 cm, and a further 1x Drop infantry and 2x Sentinel squadrons within 30cm. Opposing these forces, the Eldar had 9x formations fairly tightly packed inside their 'castle', or 15:12 ratio. This meant that virtually every Elysian activation was directed at the 'target area' and so kept up the pressure on the Eldar, even though the main Eldar formations were able to break out of the cordon to some extent.
In the 2nd game, army composition and a more cautious strategy dictated that fewer 'front line' forces were available, but even so there were 3x infantry and 2x Sentinel squadrons opposing 1x scout, 1x Assault Marine and 1x Devastator formation, or 5:3 ratio. But, the Valkyrie support could not reach the intended targets in turn #1, and the strike a/c could not hit the few AV in the deployment area without entering the lone Hunter's AA umbrella (a veritable suicide mission if ever there was one). Consequently the Elysians could not keep up the pressure. Equally, the 3x ThunderHawk transports were much more effective than the 2x Night wings at suppressing the Elysians.
However, the lack of cover near the enemy was probably the most important difference in the 2nd game. This restricted where the Elysian infantry could hide before the attack (unarmoured infantry without even AV to hide behind are sitting ducks in the open). This in turn restricted the number of formations that could reasonably be commited.
Possible formation changes (for next time)I am less sure that a single squadron of Lightning interceptors would have had that much effect on the game. Sure they would possibly have downed a THawk per turn, but that would not have turned the tide fast enough. The main problem is that they are really too expensive at 300 points (but we have had that debate before

)
I was hoping the Valkyries would provide the 'ranged' fire, and provide somewhat tougher formations to anchor assaults etc, but the Marines didn't come out to play. A squadron of Vultures is certainly worth considering in that respect. Also, storm troopers instead of a Sentinel+Valkyrie for added punch.
So, with these in mind the list looks something like this:-
HQ Drop infantry + 2x Mort 325
Drop infantry + FS         250
Drop infantry + FS         250
Drop infantry + FS         250
Drop infantry + 2x Mort     250
Drop infantry + 2x Mort     250
Sentinel                 125
Sentinel                 125
Sentinel                 125
Sentinel                 125
Sentinel                125
Vultures                300
Lightning Strike           200
Lightning intercept        300
                ÂÂ
Total 3000 points, but only 14 activations, with the single Vulture formation feeling very lonely on the table at the start.
(modified the typos on costs)