When someone lands on your appstore page they've already found your app via ads, search, browse etc.
In this scenario what would be your best course of action to maximize the likely hood of app download?
1. Going with a video from the start?
The question that really has to be answered is: does a video sell your app better than screenshots?
For an FPS game, having a preview video is most certainly a must. A 4X title which has mostly static art to show off? maybe not so much.
In short go with a video from the start if communicates your app's selling points better.
2. If so, portrait tends to get better results than landscape. Better to ignore landscape straight away?
Portrait videos do not tend to get better results.(Maybe you sampled a bunch of top grossing apps and noticed this trend. But did you take into account the orientation of the app?)
Generally the rule of thumb is orientation native to the app generally performs best . If the app is in portrait then preview video should be in portrait. If the app is in landscape then go landscape. No, don't ignore landscape especially if your app is in landscape.