Reposting the list of Shadow World adventures for sale here, with sold books removed and prices updated. I also have almost every MERP book published (both 1st and 2nd ed. - if you are looking for something in particular, send me a PM or reply here). Thanks for looking!

Nomads of the Nine Nations - G - $25
Sky Giants of the Brass Stair - VG - $30
Islands of the Oracle - M - (still in shrink wrap) - $40
Jaiman: Land of Twilight (1989) - NM - $25
Cyclops Vale - NM - $20
Kingdom of the Desert Jewel - VG - $30
The Orgillion Horror - G - $20
Star Crown Empire - M - (still in shrink wrap) - $40
Tales of the Loremasters - F - $15
Tales of the Loremasters Book II - F - $15
Demons of the Burning Night - NM - (still in shrink wrap but wrap tore in one corner and cover got a small scratch) - $35
Journey to the Magic Isle - G - $30
Quellbourne: Land of the Silver Mist - VG - $35

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.

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

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

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

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

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.

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

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.

It is unrealistic to expect Ironcrown to commission new covers for the books at this point. The most important priority must be to get all the books out to satisfy those customers who was interested enough in the books to buy them with the current covers.

Considering how long we can expect RMU to in print it seems likely there will be an updated version in future with errata items fixed and some of art replaced, but starting that process now in the hope of a few extra sales seem like madness.

Some updates on my progress:

I've recently found copies of Iron Crown Quarterly issues 9 and 11/12, so the only one I don't have is #5.

My updated list of dated product catalogs is as follows:

Spring 1987
Winter 1988/1989
Fall 1989
Summer 1990
Winter 1990
Winter 1991-92
1993 No. 1
1994 No. 2

If anyone has leads on or extras of anything not included here, please message me. Thanks