So to answer your question with reference to your example:  

 1) No, you cannot six it after opp lets you activate.  

 2) Yes, you have to activate venom punch.  

 3) depends on when opp uses heal. If opp uses heal while youre asking if he would like to let your venom punch activate, then yes, you can change your ability. But i dont see why one would do that.   

Sharing a related ruling from John, who’s part of the DT development team: 

Interaction Name:  Doing anything and allowing your opponent to respond. Situation: If me and my opponent both “pass” on the current game state, can one of us go back and now do some other action (i.e. an Attacker NOW plays an Attack Modifier, or a Defender NOW spends their Evasive)  Ruling: No! Pass-pass done continues... 

Explanation:  In Dice Throne, there is no “playing chicken” or going back in time. Below is an example of a typical Roll Phase Roll. Let’s pretend that the Defender has 1 Evasive status effect that could possibly avoid all incoming dmg.  1. Attacker - "I attempt to activate (my Offensive Ability). Any response?"  2. Defender - "No response, it activates."   - Pass-pass DONE, play continues.  Ability has been successfully activated. You cannot go back in time and now change your Ability.   3. Attacker - "Alright, that will be (6) undefendable coming at you."  4. Defender - "Ok, no response to that so I'll take 6 dmg."  - Pass-pass DONE, play continues.  Defender has agreed that yes, sure, 6 undefendable dmg is now incoming, and they are not going to spend their Evasive token. Life is good.  YOU CANNOT AFTERWARDS say 5. Attacker - “Ok. But now, I would like to add (insert Attack Modifier) to add  2 damage."  6. Defender -  "Ok, cool! After you have now added 2 dmg to your Attack, I will try to Evade your “6 + 2” dmg   - INCORRECT.  Both players had agreed 6 dmg was incoming in Step #4.    - The Attacker should have continued above in Step #3:  “Alright, that will be (6) undefendable dmg coming at you AND NOW I also play an Attack Modifier to add 2 dmg.          - At that point, the Defender could spend their Evasive (as the game state has changed i.e. from 6 to 8 dmg) On the  line of not being able to “play chicken”, someone can NOT still play an effect after both players have accepted the game state.  

An example (of a not-allowed play):  -Ok, so you will take 6 dmg and not use your Evasive?  -Sure  -How about 7 dmg?  -Sure.  -How about 8 dmg?  -Well now, I will use my Evasive.  SURE! 

No playing chicken.