I will answer what I can:
1. Celebrity says that their pre-cruise discount for wi-fi is greater than what you will find on board. I believe that to be true the few times I have checked. Additionally, since you are a repeat cruiser (and should be in the Captains Club) there is an additional discount that is offered--I think 10% in your case. In order to get the loyalty discount on top of the pre-cruise discount, you need to call the Captains Club (1-844-418-6824) because you cannot get your Capt Club discount on line.
2. In general, you can make adjustments to your specialty dining reservation once you get on board. I do this all the time, however I have never booked the discounted first night, so I don't know how full it gets and hence how much flexibility there is to make changes. I recently read in this forum that the first night gets filled so the discounts must be working. Maybe someone with more experience can comment.
3. Yes, you should be able to pay your gratuities ahead of time. Perhaps if you end up calling the Captains Club to get your wi-fi discounts, you might be able to ask about paying for your gratuities and I would imagine that they can take care of that, too.
4. You should be able to see your OBC at the top of your Cruise Planner (click on Plan My Cruise). Note that any OBC you are getting from your travel agent is giving you may not show in your online OBC total because Celebrity can't "see" it.
.5. In the distant past, my wife and I regularly brought cans of sodas on board Celebrity ships. There was never a problem or a limit that we were aware of. We did not ever need or try to get sodas brought on board from a port (some places, like Europe the formulation of soda is different, like Sprite, and I don't like the resulting taste). We stopped bringing sodas because we get it from our drink packages.