The initial problem you had with filament coming out of the threads at the nozzle is caused by improper seating of the heat break to the nozzle. In a "from the ground up" installation, you'd have an empty heat block, containing your heater core and your thermistor. Threaded into the "bottom" of the block is the nozzle, just a turn shy of being flush with the heat block. The heat break is the thin threaded segment extending from the heavily finned heat sink.
The heat sink/heat break combination is threaded into the heat block until it contacts the nozzle, at which time, the nozzle is snugged into place securely. This keeps continuous the filament path from the heat break to the nozzle. Somewhere in time, a gap opened between the two.
When you have assembled everything (including the PTFE liner), you'll want to heat the extruder assembly to about 250 °C and re-snug the nozzle to the heat block and heat break. Hold securely the heater block, as you do not want to apply force that will snap or otherwise damage the fragile heat break. Use a wrench that fits the heater block without contacting the wiring. Use a wrench that will keep your fingers safe, as the heat block will be hot.
Stepping back in time a bit, when you remove the assembly, you should be able to determine the necessary length for the PTFE tube. I checked the manual for your printer and it is lacking in detail for this information. The diameters you've specified are standard and you should be able to locate a suitable substitute from many online sources. Amazon, Matterhackers, eBay, etc.
Examine the heat break tubing. The diameter should not be so small as to allow you to push the PTFE tubing in from the heater block side, unless you have an unusually manufactured product. Dremel may have decided to create a new bit of engineering, but I'd expect not.
You'll purchase more PTFE than required and examination of the upper portion of the heat sink should give you a clue how much to use. When the cover of the extruder assembly is removed, is there a guide for the filament to make it easier to push through the PTFE tubing? If so, the length of the PTFE is from the bottom of the guide to the bottom of the bore of the heat sink/heat break assembly.
Photos of the upper entry to the heat sink/heat break, with the cover removed, would be useful, but you may have sufficient resources in hand to resolve your problem, once you replace the nozzle and purchase PTFE tubing of the correct size.