Hi Ellie,
I found some solutions to your issues, see below. Please, let me know if you have any questions regarding my steps and I'd be more than willing to help out.
1: (Regarding the extent of your map and not needing to click the home icon)
-When at your map on the first-page click settings>extent
- This will take you to a pop up that will allow for you to either- draw, zoom etc. on your area. When you select your extent and then save--this will be the extent that the user will see instead of needing to click 'home'
2: Thumbnails (For thumbnail do you mean the image of the map or the carousel thumbnails?)
-For the maps initial image this is can be edited at 'My Stories'>click on your map> upload thumbnail see below
If you are referring to thumbnails in the carousel I have attached a link with highlighted information that gives you a good idea of what is the recommended image sizes for Map tour
Thumbnail Information-Recommended Size
3: Video
For 'Map Tour' layout here is the link to explain videos in Map Journal: Highlighted on the page
Video in Map Journal
Also here is a walkthrough for your template that helps you get a video in map journal
Walkthrough for adding video in Map Journal
Again, if you have any further questions about the information I've provided feel free to reach out.
Best,
Luba