Temp bare wires

I am moving a fireplace approximately 24 inches from it’s current location. The original builder ran electrical wires approximately 10 inches or so above the zero clearance fireplace that spans a corner of a room. (Seems wrong to have electrical directly above a fireplace, though I digress.) Anyway, after moving the fireplace to its new location, the chimney will now run directly where the wire is currently. I am planning to move to wire to the outside wall, rather than the wall that acts as the face of the fireplace, though there is not enough slack to run the perimeter of the wall. My thought is that I will splice new wire in place to extend the length and run it well away from the fireplace. Keep in mind, there is not enough slack to run it anywhere but where it currently is. My question is, can I splice the ends together with wire connectors in 2 metal/sealed junction boxes? The only problem would be is that the junction boxes would then be covered with drywall providing no access point. I am not concerned of getting back to the junction boxes, but performing the safest and best method of lengthening the cable. If there also is a better way I welcome any thoughts. Lastly, I will also have to remove a recessed light in the ceiling by dry walling over the hole. Actually, the wire will be extended to other lights being controlled from the same switch. Can I split the new wire within the existing canister and run it elsewhere through the ceiling? Again, the question being is it OK to drywall over a wire nut/connection point (canister)? Thank you.

John or anyone, My son and I started a 100' trench from the house up to the new pond. I've got 100' of 12 ga. UF cable, carflex for each entry point and a weather-proof 4 outlet box that will be att'd to the GFCI outlet in the front of the house I also have an auto-fill valve for the pond to compensate for evaporation. Here's my question: Can I put 100' ft of 1/2 inch black poly pipe in the same trench with the uf cable?? I read somewhere it is okay, but just need reassurance. fred