Hi,
You have raised a few good questions. One of the guys at our club has a 100" version of this plane. He uses it as a glider launcher from a frame on top of the wing.

It will be fine to move the engine forward of the suggested distance. It may actually be better anyway as it will assist with the CG balance. What I suggest you do is temporarily set up the engine and have all hardware in plane (servos, battery etc) and with wing on. Check CG. I wouldn't be surprised if you need to get more weight up front anyway. If you do, just move the engine forward about 1cm and see how you go. You can then resort to moving around battery and receiver to get correct CG. Saves adding any dead weight.

In regard to the engine secring bolts, just use the nylon insert lock nuts without locktite. You can also use metal self tapper screws. They won't vibrate loose in a hurry.

Check your CG and Lateral balance (side to side). Balance as necessary. If needed, try moving the engine even further forward. It won't create any major drama by ding this.

The gereral rule for the tank is to have the centre just under the level line of the carby. I havn't had any probs with tanks syphoning to the carby and I have a few engines set up where the carby is 1 to 2 inches lower than the centre line of the tank (P47 for example with an inverted 1.20FS).

Look at buying a CG machine or make your own. Use two dowls with pencil rubbers on the end. Set up in a block of wood or on your work bench. Works fine.

By spending a bit of time setting up and getting your balances right, you will enjoy your flying more and the inevitable will be delayed. Don't be too concerned about the changes with the engine position. Again, try and avoid adding dead weight.

Regards
Mark