A: As you acknowledged yourself, your tension is the problem. Screams usually sound huge and brutal, like there is a ton of effort behind it, but that's not the case. First off, you have to realize that an unamplified scream is going to sound a lot different than one done with a mic with some EQ, compression, and effects added to it. Keep that in mind if you're not able to practice with a mic. Once you do get that opportunity, you're going to be shocked at how true it is that a little bit goes a long way.
None of thats helpful though if you can't get the scream sound going in the first place so here's what you need to try. First off, read my article on screaming to get a more in depth understanding. Basically, warm up as you usually do and sing through a few songs to make sure you're actually warmed up. Experimenting with your voice is fine but you need to be sure its actually ready to go and can handle the possibility of straining a little while you work towards the proper coordination. Sing an 'AH' vowel, as in father. An open vowel is a lot easier to rasp/scream when you're first getting started.
As you're sustaining the AH, push down like you're going to the bathroom for support and let out a little more air. This air needs to be directed toward the back of your throat. To get this sensation down, try imitating a pigeon. By directing the air toward your uvula you're going to start getting that pigeon sound as it vibrates. Now... for the AH vowel you want to direct the air into that some place but this time with just a little more pressure. Keep trying this with different mouth shapes, tongue positions, etc. until you start to get a rasp sound. Does it tickle? If so, back off the pressure a little bit. If not, try it either at a higher volume or with just a tiny bit more air pressure. Then back off the pressure and see if you can replicate the sound with a normal amount of air but still focus it up high.
Play around with this, always backing off if it tickles, hurts, or you choke. This is the first step to really getting a feel for where rasp goes. From there you can start to build on it and imitate the artists you mentioned. Feel free to contact me for more detailed information through a written consult or in person lesson!