You say it also leaks oil when the engine IS NOT RUNNING.
If so this should be easier to locate the leak.
Might could use a dye in the oil to locate the leak.
Use about 1/4 oz of motor oil dye. You can review such on-line.
I did not read all of your post, but if you will get the EXACT model number from your mower, usually from under the seat and download a free owners manual it usually has all the exact steps for replacing belts and adjustments for the GT series of tractors.
Instead of using a jack for lifting...
Here is a link where you might find manuals and wiring diagram for your Cub Cadet LTX1040 EXACT model number.
https://www.partstree.com/models/ltx-1040-cub-cadet-42-lawn-tractor-parts