The 'problem' is pretty obvious. ICE isn't in a position to be paying higher end artists. Period.

I don't recall such a feature. I can't guarantee that it might have been an unexpected side effect that is no longer available. The idea for existing items is that you click on "Add missing default items" and work from there. And if you don't want to use the complete default item list, you can always edit ERA/RMC/Adventures/Default.inventory.XML

That works in the opposite direction. Items added to the individual character inventory are automatically considered when checking the party inventory.

You can't delete items in the party inventory that were added from the individual character inventory. This is on purpose, since the character inventory is not saved when saving the party inventory.
The idea here is that gaming groups would normally only use one or the other method for managing inventory. The party inventory lists all character inventories so that it can be seen then in the Character Status module. If you are playing with items managed during the adventure, you'd be better off directly adding them in the party inventory.

I didn't put any examples for that in the RMC ruleset. The idea is that you can define a maneuver table that is eventually used with a specific skill bonus in mind.
Check the equipment repair table in RMU. If a character wants to roll for Leathercraft, Metalcraft or Woodcraft they will get the option to use the Crafts table due to the category of those skills, or the specific repair maneuver table.

Rather than choosing the mana source at time of casting, I would have a character with multiple Tap <whatever> Mana talents choose which kind of mana they were using each day, and no switching once the decision has been made for the day. Keep in mind that it is expensive to learn multiple types of mana tapping, as you only get one for free. Keep in mind that Granted and Personal are pretty much the same except that the former can be denied if the gods or just the multiverse are mad at you.

I run into some problems, not really sure if they are bugs, or I'm doing something wrong. I remember few updates ago when I was adding an item (either in the character module or the inventory module) in the reference field, when I wrote the item name, it was automatically adding the item from the default items list, meaning it was adding some description notes as they are in the original books, but it seems it's not happening anymore, it was specially helpful for herbs and such.

Also, when I add items to party members in the inventory module, they don't appear in their character sheet :/.

Besides, there is any way to remove items from specific party members in the inventory module?

In the last update is added a new feature, -Added support for maneuver tables related to skills. Where can I use this feature?, I can't figure it out