Using liquid cement will soften the rubber DS material, but that can be a two-edged sword. To a limited degree you can sand the material, but it doesn't sand smooth.
I haven't tested this theory completely, but using the LC after some sanding may work.
Honestly, when I can, I replace the rubber-like parts with Styrene or Resin parts as they are tricky to deal with.
As far as the tracks go, you can simply glue them. I made the mistake of clamping the first set of DS tracks I came in contact with and that caused them to squish and distort on the side I clamped. You can use a few ounces of weight ad that should be enough to keep the ends together, but make sure you think about where you would want any odd marks so you can hide them easily later.
I highly recommend after market plastic indi-link tracks for the most realistic appearence (on an M-48 anyways). They will require tedious assembly and clean-up, but they look the best. They make it easier to test fit your tracks to get the best fit as well. You can take them off and put them on at will and when you go to put them on permanently, just add a bit of liquid cement and they stay right where you put them.
Honestly they are a pain in the ash, but I really believe they are worth it.
For the mantlet covers I can only say that you should proceed with caution unless you have a spare. A sharp blade will help, but you have to use a sawing action to cut out the flash.
Trim, sand, and try the LC to soften up the areas you work on. But be sparing with the glue and take it slow.
If all else fails, then I hope someone has a verified and tested method that works perfectly every time before you have to try something risky.