Some questions about these cards:

1) Do I immediately open the Chest from the Dungeon expansion without take it with me? (similar to a bag of gold)

CHEST (Dungeon)
"You may roll 1 die to open the Chest. If the result is less than or equal to your Craft, gain 3 gold.
If the result is higher than your Craft , you are jabbed by a poisoned needle and lose 1 life . The Chest then crumbles to the discard pile."

2) ...Can I do the same thing with these Treasure Chests? It can be usefull if you have 4 objects yet and you want to draw a trinket or just for gold.

TREASURE CHEST:

A - You may discard the Chest during your turn to draw the top 5 Adventure Cards and take 1 Object of your choice. Discard the remaining cards. If you do not draw any Objects, gain 3 golds instead. If you have the Treasure Map , you may draw 10 Adventure Cards instead of 5.

B - You may discard the Chest during your turn to roll 1 die and gain that much gold .

If you have the Treasure Map , you may roll 1 additional die and gain that much gold .

C - Roll 1 die to open the Chest:

1. Trap; lose 1 life
2. Trap; lose 1 life
3. Gain 2 gold
4. Gain 2 gold
5. Take any Object in the Purchase deck for free
6. Gain a Talisman

Once the Chest is opened, it crumbles to the discard pile.
If you have the Treasure Map you may add up to 3 to the score.

D -Roll 1 die to open the Chest:

1-2) Trap! lose 1 life . 3-4) Treasure! Gain 2 gold . 5-6) Ancient Relic! Either take the top Object in the discard pile or gain 1 Talisman .

After the Chest is opened, it crumbles to the discard pile.
If you have the Treasure Map you may add up to 3 to the score.

3) If I have two or more Treasure Maps, do the effects of Treasure chests A-B-C-D stack? Example: add 6 instead 3 to the score, draw 15 cards instead 10, roll 3 dice instead 2.

4) Why the Treasure Map from the Remnant deck (Cataclysm exp.) is a Trinket ? Do you think they missed to write the "trinket" keyword in all the other Treasure Maps?

Thanks!!

The way we have always played it is that these cards are all "objects" therefore they must be in your possession (i.e.: carrying them) in order to use them.

1) Do I immediately open the Chest

No

2) ...Can I do the same thing with these Treasure Chests?

No

3) If I have two or more Treasure Maps, do the effects of Treasure chests A-B-C-D stack?

no

4) Why the Treasure Map from the Remnant deck (Cataclysm exp.) is a Trinket ?

Who knows. The y did that with other objects too across expansions.

5) Do you think they missed to write the "trinket" keyword in all the other Treasure Maps?

Trinkets didn't even exist prior to Highland.