If I have SI Dr Strange with an 18 DV and Defend next to him is GSX Prof-X with a 16 DV. Prof-X shares the Dr's 18 DF can I then use the Dr's perplex to up Prof-X's DV to a 20.
Thats replace then modify correct?

Totally legal.

Now if I had a second perplexer it could go as high as a 21 DV correct?

Yes. No higher though.

That's easy. Just push the Professor.

Push the professor THEN perplex

Because otherwise, as soon as P-X takes the damage from the push, the perplex goes away (happened to a friend of mine)

Actually you can go as high as you want. 22, 23, 24

See how high you can go. Then, when it comes time to use your Defence value, it becomes value +3 max.

Sometimes perplexing more than 3 (i.e. Doc Ock with 4) can come in useful. You perplex all onto defence and stay in hindering. Opponent has two police. They modify the opponent's defence by -2 using their TA.

Perplex up 4, +1 for hindering, -2 for police TA... ah nertz, it is +3 and that is legal!

Even better use Armor, put "Alpha" Oracle on ICWO, and make Xavier an agent so he gets Perplex, Mind Control, & Energy Shield Deflection. The final touch Elite Sniper so Xavier can continue to play battlefield general no matter how many opposing charactors are adjacent to him. Oh sure Mind Control might not be the best power but first turn Perplex & the +1 to combat values right before a Psychic Blast is just money

Elite sniper would still only allow him to make the ranged attacks against adjacent figures.

That's not to mention that you're also talking about a game at 400 points, minimum.

That's right which is far better than Xavier being not able to make ranged attacks at all. At least now he has the option of Psychic Blast which should make opponents think twice before basing him

Down the stairs? Be sure the insurance policy's paid up first!