I want to get rid of all my splitters and have the most direct connection to my modem. I was going to use a coaxial double male ended coupler. Am I legally allowed to access my cable box to do this? I live in a house where a line comes in from the street via a large orange cable and my box is held closed by a twisty plastic thing that I can easily undo.
In the US house wires are the CUSTOMERS property and responsibility.
The cable company can install them but they immediately become yours, as in next week you can cancel them move to an antenna and use them or directv or whomever. You do run a risk of being charged when you mess things up, just like if you broke a pipe and did not know how to do plumbing. It is going to cost you, and you should consider that before proceeding.
I will advice you as best I can, barrels or F81s are 3 pointless potential points of failure [2 fittings and the F81] if you can avoid using them you should. Granted you can use them, but simple is better.
If the outlet you are using will reach the cable companies ground block, that's where it should go. No matter what you think you cannot put them on properly using your fingers. You need a 7/16" open ended wrench or a pair of pliers. Fittings are made of cheap metal, you can break them easily don't go putting your shoulder into it, just your wrist and just snug.
If you remove splitters the signal levels on the modem will change. you need to figure out how to log into the modem and view them and make sure the change is not going to make you out of spec. The upstream will go down, and the downstream will go up when you remove the splitters by whatever the splitter is rated for [2-way by 3.5 db, 4-way by 7, or 8-way 10.5]. You want the DS signals to be -10 to +10, and the upstream to be between 35 to 45, they can be lower and work, but this is the ideal levels.
As to stealing services, most cable companies are digital and encrypted which requires a box at a device so stealing cable is not like it used to be.
The bigger risk is you do it wrong and cause interference on their network so they filter you out to prevent it, reducing your service level. Keep the stinger straight, use the wrench to snug it, and this will minimize the chance of that.