As noted, carbon tet is one of the nastiest of industrial solvents. It is a known carcinogen, and can cause brain damage if inhaled in high concentrations; and it can cause renal damage if exposure is prolonged or repeated. I'd say there are much safer materials to do just as well.

The thing that causes strings to lose their "brightness" is mainly that they get lumpy. It's not so much that there's a bit of grunge on them, although of course that doesn't help; but the gunk isn't distributed perfectly evenly, so you string has lumps that prevent it from producing perfectly harmonic overtones. The "lumps" can occur as local deviations in string density (weight) and/or localized deviations in "stiffness."

The two opposite methods you can use are to get them scrupulously clean, and remove the lumps, or "fill" them with a uniform impregnant that spreads evenly in all the nooks and crannies.

Boiling them in clean water may help, if the grease and grime is made fluid enough to float to the top. What doesn't float off may also be rendered fluid enough to "even out" a bit. Let the water cool enough to skim off the top water, or "decant it," after you've boiled for a while so you don't draw the strings back through a layer of floating crud. (The surface layer will be there, even if you can't see it.)

Adding soap is likely just to put more grunge back, and if it gets into the windings may be very hard to remove.

Soaking in kerosene should have a similar effect since the kerosene should penetrate and will dissolve most "body oils" - if that's what your lumps are - and, if it's a good grade, will evaporate almost completely, although it may take a very long time to get it all out of the spaces in the winding.

If the lumps you've got are from localized "necking" of the core wire, or from displacement of the windings, or from internal corrosion in wound strings, there's not much hope of a significant restoration, and since you can't expect to remove corrosion and replace the "finish" to an original quality, no restoration is likely to last very long.

You can also just tell people you're a "folk artist" and you like the softer sound of old strings.

John