3) If changing a fork, you should aim to have the same effective length when riding. If your fork ends up longer, its able to exert exponentially more force on your head tube.
Also, a longer fork will raise the front of the bike effectively slackening the head tube angle and giving you more trail. The rake angle of the new fork may also add or reduce trail. Either way, yes changing fork is likely to change handling.
Ideally you want the bike to have the same silhouette from the side before and after changing the fork.
Breaking that down, you want the same trail measurement before and after to keep handling the same. The top tube should be the same angle from the horizontal before and after, and the wheelbase should be the same.
If you fit a longer fork with the same size wheel it will look more like this. Note this is 2 degrees of rotation centered around the rear axle.
The upshot is there is now more trail, so your bike will resist turning more, until it gets to about 25 degrees of steering input and the front wheel will then try and turn sideways hard, because it wants to "fall over"
Upshot, you want the trail value to be identical before and after, and ideally the forks no longer than original.