1. 1 probably as most pc ports that originated from other devices (handhelds/consoles) are usually terrible ported after the fact.
2. there is an export option from VS for iOS & android, but I don't think they are working too great in the current build? Simon? But you can always wait for the next update of the engine or put in a request to the devs to see if they can export you a working iOS build.
3. I would say something around 1280x720 if you want to optimize the game better to run on both pc & iPad smoothly. Also you will probably be wanting to reduce loading times & required hdd space lower as well & to do that I recommend using XnConvert (a free batch image converter) to convert all of your image files to webP format. See here
for more on recommended image formats & XnConvert & see here
for information regarding game optimization with Visionaire Studio.
4. I vaguely recall something mentioned in the changelog of one of the engine updates a while back mentioning mobile gestures, but I think it was only accessible through Lua script. Simon will have to confirm as I've never had reason to use let alone look into them.
P.S: welcome to the VS community.