It's entirely doable, but there isn't any support within the sex system for those particular changes and some of them can end up being rather complex to implement. I would rate this as a significantly difficult set of changes that would probably take weeks or months for most people to implement.
1. Orgasm is a function in newsexsystem.gd. The only challenge is coming up with a proper set of conditions to check as there are some variations on the conditions you gave, such as the double dildo actions.
2. It's not particularly difficult to append the last action to the list of ongoing actions, but sex actions effectively have no state information, meaning that they don't track any changes or variations. The sex action scripts are reused by other participants so you cannot easily add variables to the sex action files to track the timer for a specific act. Storing the timer outside of the action and using it to switch between two types of text generation functions is complicated by the alternate text system that is used by many penetration actions, but at the very least this means altering the every penetration sex action file to handle the unique wording. I expect that altering the following line in newsexsystem.gd to include an entry for your action state information is the simplest approach:
var dict = {'scene' : scenescript, 'takers' : takers.duplicate(), 'givers' : givers.duplicate()}
3. There is some precedent for disabling action buttons based on other conditions and the conflicting actions function might produce data that can be used to accomplish this. Though there are other considerations for preventing actions from being cancelled, such as the "Interrupt" URL for ongoing actions. The sex system's GUI currently does not support alternate uses for URLs nor showing tooltips, though such functionalities exist for other GUIs in the game. Also, all actions get cancelled when restraints are removed from a slave that is resisting sex or when the sex interaction ends.
4. Timers themselves are easy to manage, but going back to #2 the core problems are where to store them and share them.
5. This is covered in #2, but changes to the flavor text of sex actions is messy work and likely to consume a disproportionately large portion of your time.