Something came up during a game last night concerning the swapping + modifying of DVs.

So textbook case: FF Susan Storm uses the FF ATA to lend Johnny her 19DV. Johnny has ESD so that makes gives him an effective 21DV from range, correct?

The actual situation that came up was Rampaging Hulk lending his 18DV to Ultimate Green Goblin who was holding the +2 Office Desk. Would that mean UGG has an effective 20DV?

Taking that further, if two characters were using TAs (eg. Batman Enemies/JSA) to swap AV/DV could that target character's stat be increased (eg. Perplex, Hydra) after the fact?

Under today's rules, it all depends on when the modifier takes place. Everything you list above - except perplex - takes place with the attack, so you would replace and then modify. Perplex, on the other hand, is its own action, takes place before the attack, and would therefore be overwritten by the replacement.

With the Blackest Night ruleset next month, the concept of replace/modify changes so that the timing does not matter - all replacements and modifiers are calculated at the time you need the combat value, so even perplex would be available to the figure after replacement.

Correct.
Right again.
At present: yes to Hydra, no to Perplex due to timing issues - the Perplex bonus is applied before the attack is made and is then overwritten by the replacement value.
Once the Blackest night rules go into effect: Yes to both.