If you can't combine the swords, I recommend you use them in tandem (together). Use the fire aspect sword to start the combat and deal damage over time, then switch to the sharpness sword and slice and dice. Once he/she is no longer on fire, hit them with the fire aspect sword again.
Occasionally I test stuff out before but most of the time my attitude is more to the tune of "WEEEEEEEEEEE! LETS BUILD SOME STUFF!" That's worked out pretty well so far. I'm very efficient at gathering resources so it's not a huge problem to waste a few hits of a diamond tool to undo something.
Tunnels branching off into more sections of the base is definitely a good idea, As for the door design, i recommend some sort of 4x4 tunnel with stairs in the corner as it will feel nice and spacious, For the main door, I think a piston gate is a good way to go, if you know enough about redstone to do it.
Strip mining on Y=11 with 3 or 4 blocks between tunnels is widely considered to be the most efficient way to get diamonds and the like. It is on top of lava and all blocks revealed have higher concentrations of diamonds.
The only part of this I would ever support is a damage increase from high velocities in the direction of the target (so hitting someone while charging towards them on the horse hurts more). Otherwise, its just going to be annoying. Minecraft is more the sort of game to bash people apart with a sword mindlessly.
Strip mining at or below Y=11 (2 high tunnel with your feet in Y=11) yields the highest amount of diamonds, with diamond concentration dropping sharply at bedrock. Y=11 is often considered the best layer to mine due to that it is level with almost all lava pools, and you can simply bridge them over with a water bucket.
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A. underground
B. cheap to build
C. easy to design/understand
Personally I use a sky level mob spawner with water flushers and a villager to help clear out zombies.
EDIT: Quoting didn't work for some reason. I was quoting Toms007
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