1. When you go that far out, how do you determine if you have a headwind or tailwind? And cant that change at various points (and elevations) in the mission?
Before you head off into the distance, you always need to understand what the wind is doing and how that is likely to affect your flight.
You should be able to judge wind speed and direction at your flying location by observing trees, flags, waves etc and feeling the wind on your face.
If the direction and strength cause concern but you aren't sure, you can simply put the drone up and try flying into the wind and see how much that affects your speed.
You know how fast your drone goes in still air (16 metres/sec with OA disabled).
If you can only fly at 5 m/s against the wind, that means you could fly a short distance away and get back slowly but would have a hard time returning against the wind over a significant distance.
And/or you can observe how your drone hovers in the wind.
If it can't hold position, you don't want to go far from home at all.
You also need to understand that winds higher up will be blowing stronger.
2. When flying BVLOS, isn't turning Obsticle Avoidance off a little scary (if not unsafe)?
It doesn't matter how far or close the drone is, you always need to be aware of the flying environment and what obstacles are present.
Obstacles are involved in most crashes.
It's very hard to get into trouble when there's nothing to hit.
If you fly over the sea or open country at a height where there are no obstacles to worry about, it's pointless bearing the significant speed (and range) reduction that obstacle avoidance puts on you.
And generally if you think you need OA, you'd be better off flying further away from the obstacles that are scaring you.