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In a normal year, the wind blows primarily out of the NW in Spring and the SW in summer in Seattle, owing to the normal position of the polar front jet stream. This year may be worse than normal for the park because the trough is stronger than normal and for much longer.
THe NW flow should theoretically produce a miniature terrain blocking pattern with high pressure building up on the NW side of the park (that's the third base foul pole) and low pressure downwind...the park is wide enough and tall enough that it should force the wind to go up and over it or around, yielding that build-up of air underneath and the lofting effect you're talking about. The only way to defeat that is to cut the inside of the park off from airflow or force the air to go around the park, rather than up and over.
Incidentally, a stable marine layer is more prone to the terrain trapping effect I mentioned because the air can't really go up (any upward motion will cause it to be cooler than the surrounding air and therefore more dense...and it will sink back down). A ball in flight encountering heavy, stable air should spin more (so if it has backspin, it will float, it it has topspin, it will fall faster)...and if it's hit to left, those effects will amplify and the updraft/pushback will be strongest.
I think putting up the roof does very little to the spin factor (dense air accentuating spin since the seams grab the air better) BUT it does cut the field off from the surrounding airflow, removing the updraft problem and reducing the wind blowing in from left. Anything that displaces the pressure difference between the left field corner and the first base side could help.