As far as the -7,000 on the lmi with it unhooked. The pressure transducers read all pressure on the boom cylinder. You have to "deduct" out of that amount, the weight of the main boom. Its applying force on the boom cylinder, but its weight has been calculated out of the load chart. The lmi needs to account for this weight, so it has to automatically take the weight of the main boom out of the lmi readings.
You can't have the jib taken out of that amount, because it is a load, and isn't calculated out of the lmi reading, because you can run without the jib mounted.
I've actually removed the jib for certain jobs where I knew I was going to be right up to chart, and needed that extra amount, but didn't need jib. You'll notice with jib removed, probably 1/2 to a little more of your 3,000 lbs preload, would disappear.
You'll also find, that your "preload" amount will be much less at a higher boom angle than a lower boom angle, that's showing the effect of the stowed jib, its effective weight will change based on boom angle.
If you watch really close too, you can see the weight go up and down, based simply on whether you are booming up, no booming but just winching, or booming down. You can pick a load up with just the winch and be just inside chart, and then start booming up to bring it to you, and have the overload alarm go off. The weight of the load didn't increase, but the hyd pressure on the transducer did when you started to boom up the load.
Just make sure you don't get carried away and remove all the preload up to the amount of just your block and ball, it has to include the stowed jib.