I will try to answer your questions to the best of my ability.
"1-How the move speed adjust works? Why it has 6 options? Why is there an increment in values from speed 0 to speed 6? How do these values interact with each other?"
The Move Speed for each selection is setup in the Basic Game Settings(Base Game Settings) of the Editor. You set how fast each selection of speed should be for Hero/Allies and for Map Events, which are separate from each other. If you are using the Sample Game Engine, one of the Common Events sets the Hero Move Speed to 3(I believe) when you start up the game. You can select the Speed for the Map Events offside, but you can also select the Move Speed for the Hero/Allies/Map Events by using the Move Route Event command.
"2-Is there a way for an item to directly call a common event on use without having a dedicated map event to call the common event first?"
There is an example of this in the Sample Game Engine. After playing through the Dungeon and defeating the Boss, you will receive the "WOLF RPG Editor" as an item. When you select the "WOLF RPG Editor" from the "Items" menu, it calls a Common Event that is set up with a string of Messages for the player to read. Within the WRPGE itself, look inside the Databases where the "WOLF RPG Editor" item is listed and see how they had set it up.
"3-Do you know if there's any plans to add the option to export games to be played in the browser in future versions?"
I don't know if the latest WRPGE Version 3(Japanese) has the option for exporting to browser, but I know that there are WRPGE games that are on the Nintendo Switch. And since Version 3 has a Free and Pro(Purchase) options, I would say aside from extra features that the Pro version has the other exports.
That is all the information I can give at this time. I hope it helps you.
Good Luck, Take Care, and God Bless.
Widderune
Jeffrey joined
Widderune edited WOLF RPG Editor English (widderune.wixsite.com/widderune/wolf-rpg-editor-english), the English translation of "WOLF RPGエディター" (www.silversecond.com/WolfRPGEditor), which was translated by Velella Himmel (vgperson - vgperson.com/). He has retired from the translation work, but he will try answering any questions regarding WOLF RPG Editor English.