I do not have access to altium and the videos right now, but I will try to answer what I can:
2) in system preferences you can add another grid. Then press G you will cycle between them. (System preferences are accessible through the wheel / gear in top right corner)
3) I think I saw something in the Advanced settings. If I remember right it could be located in System preferences -> General -> bottom right corner, there should be a button
4) I am not sure if this exists in AD18 / AD19. I think you just see all the layers which are added to the project or are not empty
6) Not big change - still similar - you only can par one existing with one empty layer. You can have a look at my youtube video
https://www.youtube.com/watch?v=KpgT...puSXz6sROQmO6R7) it will increase space between them and they will be nicely routed. If I route the with 0,2 from beginning, I would have to use a big gap rule which is sometimes not very practical. There are also there reasons e.g. impedane matching and adjustments, but this is for more complex designs
8) I believe, in original Arduino use of the forth hole is limited because the connector was extended - and unplated hole had to be used (plated would just not fit correctly?)
9) I ground them and if you find out it is causing problems you always can use plastic spacers or isolation
10) simply to say - to be able to route at least 1 track between the holes (sometimes two tracks) and still be able to solder the pins properly .. that is what I use