My 2 cents worth on your questions...
No. Keep the speaker wires and electric wires apart from each other. All wires (speaker, coax, electric, telephone, light control, so on) should either run parallel (with sufficient distance apart) or at a 90 degree angle to each other.
Also, it is advisable to put all of your theater equipment on the same phase. The easiest way to do this is set aside one circuit to run all equipment and nothing else. I have a single 20amp circuit that runs my home theater and whole house audio.
Your speaker wire should be UL rated for in-wall installation. Local building codes may dictate the specific rating required in your area, look for a rating of CL2 (typical home use) or CL3 (typical commercial use). I have Sheer Wire Oxygen-Free 14AWG speaker cable (part number 142OFC105) for all theater speakers except the subwoofer. I have quad-insulated RG6 coax cable for the amplified subwoofer. The coax cable is finished with a standard “F” connection then has an F-to-RCA converter on each end.
In-ceiling speakers placed behind the last couch should work well. Surround sounds are meant to be less directional, meaning the listener should not be able to determine the exact location of the sound.
I suggest building your raised platform from 2x10’s with 3/4” plywood on top. Glue and screw everything to minimize noise. Make sure the back of the platform has a raised edge or somehow prevents the couch from falling off. Add a pathway light on the step. Carpet/finish the platform to match the rest of the room.
I created a
calculator that can be used to determine the THX and SMPTE suggested screen size. It’s just an estimate, so don’t get hung up on the precision of seating placement. A few inches closer or further away will not matter. IMO, a front projector has the highest wow factor and makes your theater a little more future proof.
To sound proof my theater, I used a sound insulation (similar to regular insulation, but with higher sound absorption factor) between the joists and studs. On the ceiling I added a layer of sound board and then a layer of 5/8" type X drywall. On the walls I put two layers of 5/8" type X drywall on both sides. I used sound insulated doors to the room and to the basement area (where the theater is located). My soundproofing focus was keeping outside sounds from entering the theater, and was less concerned about containing sound in the theater.
and just running the wire out to the speaker. This type of plate is available at most hardware stores for a buck or two.