I'm thinking about using the Grav Well to block effective jump-ins with this objective. But I'm not sure what's the order of placing tokens would be.

Both the objective and the upgrade says "before deploying fleets". So my first guess was that I as first player with he Interdictor have to place my well token first, then the second player places the hyperspace tokens. It still could work but not as effectively. But osmeone argued my point and said that modifying the table according to the objective card would still count into establishing the play area so the hyperspace tokens would go first, and then the grav well token. Which one is right?

You're right, osmeone is wrong.

RRG pg 5 EFFECT USE AND TIMING

Quote

If both players have effects with the same timing, the first

player resolves all of his effects with that timing first.

If the Interdictor is P1, you place the Grav Well token before he places the HA tokens. P2 has the option to resolve one or the other first, but that's not really relevant since in that case you're playing the Interdictor's objective anyway.

I'm not sure if osmeone thinks he is using the rigorous definition of establish the setup area as laid out in the RRG, but that step ends three steps before Objective Selection even happens, and five steps before deploying ships:

RRG pg 10 SETUP

Quote

1. Define Play Area and Setup Area:
Clear a 3' x 6' play
area. Then, establish a 3' x 4' setup area by using the
length of the range ruler to place the setup area markers
1' from the short edges of the play area. The players sit
across from each other on the 6' edges of the play area.

2. Gather Components:

...

3. Determine Initiative:

...

4. Choose Objective:
The first player looks at all three of
his opponent’s objective cards and chooses one to be
the objective for the game.

5. Place Obstacles:

...

6. Deploy Ships:

...