A trombone is the sum of its components. And the words of the economist, Thomas Sowell, "There are no solutions, only compromises..." applies to the trombone world as well. Part of why you might like the Bach are some of the things that you are trying to "fix." Similarly, part of the reason you liked the CL2000 valve may be because you also like the Conn 88, which has a number of design elements that are vastly different than a Bach 42.
The Christian Lindberg valve is not merely a "not so wide" valve. It has a completely different design compared to a traditional rotary system. It is shaped like a "Y" and so all three parts of the inside are exposed whether it is engaged or not. This is the reason for the short throw but also a design element tha may not work particularly well on other horns. That may be the reason that "no one" has ever done this (although you may be surprised as well).
Now, the 42 is notorious for having misaligned or poorly shaped ports and cores. You might hear that the rotor is undersized because it is the same rotor as the 36. While it is true that the 42 shares many components with the 36 (everything between the rotor and the bell flare are identical on a 36 and a 42. The only difference between the rotor and end of the horn on a 36 is that the 36 has an 8" flare and the 42 has an 8.5" flare)... that does not mean it is undersized. It has a 562 bore tubing, which is the convention of other large bore horns and can operate just fine when it is installed properly.
*NOTE: A port is the round thing that the rotor casing uses as an entry and an exit for the air/sound waves. Tubing is inserted into to the ports.
That said, it is not uncommon to modify the 42 with another rotor. In the 1980s, it was very common to install Thayer/Axial Flow valves (these two terms are used interchangeably) on Bach 42s. The rotor was seen as a "chokepoint". As mentioned above, in many cases this was because it wasn't built very well to begin with and there is a gap between the slide receiver and the rotor that can cause problems as well. Reseating it can make a big difference. Installing a new type of valve system requries this so it is something that may have contributed to someone liking the change afterwards even though they might have assigned an undue amount of credit for the valve when it was at least in part the better quality installation of the new valve that made it "better."
In the US there are no as many Latzch modificaitons as it is a little difficult to import them, although the Rotax valve has been a common modification (and even installed on the Edwards "Alessi" model). The business model of Greenhoe was to take Conn 88 and Bach 42 parts fromthe factory, assemble them with care, and put their own take on a rotor as well. They have since retired, although Schilke purchased the rights to the company though as far as I'm aware do not do conversions as Greenhoe originally did.
Another option are instrument innovations rotaries. Their thayer valves are the same as the ones on the newer Bach 47AF ("Infinity Series") Thayer valve trombones. Both their rotaries and thayer valves have bearings that they "float" on instead of making direct contact with the metal so they have a very good action and seal well. They are relatively new so their prices are still very competitive, although if you have to import them it might be cost prohibitive.
Now, with any of these replacements, you will likely have to unsolder the tubing and adjust for the placement of the new valve. Not all of them are "drop in", or in other words not all of them have the same exact shape as the 42. Especially if you are going for a larger rotary valve.
Lacquer is 100% unnecessary for your instrument to function. In fact, there are many professionals who swear by removing the lacquer from their instrument as much as they can. It is a common modification to remove the lacquer at least from the bell and/or any area that does not directly touch your body. The lacquering process is very labor intensive, and consequently expensive. In my experience of selling instruments, you never recoup the value of the procedure. If you want to spend the money because you want a shiny instrument, by all means, if you can find someone who will do it well then go for it. But don't do it thinking that you need it to make the horn mechanically viable or that you will increase the value of the instrument.
I'm not sure what you mean by "the main material is just before corrosion." If you mean red rot, then I wouldn't put any money into this instrument depending on how bad the damage is. You can have someone buff out the damage on say, the bell, though as I mentioned above, you do not need to lacquer over it. If you have a problem with the outer slide or inner slide, you also dont' need to lacquer that as the inside of the instrument is not lacquered on any instrument and that would be where the damage is coming from. You may need to replace a tube or something in the long run, but you would be fine just using it until such a time.
This is totally subjective. I've done this procedure on several horns. Actually, I installed a Shires rotary on a Bach 42 a few years ago and was very pleased with the result. (Although the horn did not have a rotor prior to me adding it). As I mentioned, the cost of it being shiny is purely in the realm of the subjective. YOu should find someone to lacquer it who knows what they are doing though. It is easy to put too much lacuqer on and completely deaden the instrument.
You may also like the way the horn plays after swapping out the rotor. BUt its also possible that having your tech align the tuning slides and remove burrs from the tubing that you like the way it plays as well. Its hard to say without actually going through the process. I can say that if your tuning slide isn't moving that you probably have some alignment issues which can really cause a horn to feel like it is fighting you when you're trying to play it. Merely getting those fixed may well be sufficient.
However, if you go with another valve, you will have to get everything aligned anyway in the process. It will save on labor cost to have that done at the same time, though the process will be more expensive overall this way.
In my personal experience, I would probably have the bell section disassembled and then reassembled properly also specifying to have the ports aligned and shaped properly.
Other common modifications to the 42 are to get the "M" neckpipe installed. As mentioned above, the 36 and 42 share a neckpipe that is the same size. Many have installed the Bach 42M neckpipe, which is the more "open" version of the neckpipe and have reported good results.
I've personally had the tuning slide "reversed", which means that the tuning slide has a female and a male side, not male + male. This gives a slight increase in the amount of conical tubing and made the high register respond more easily on my instrument. However, the tuning slide was stuck previously, so how much of that is due to the fact that it was in alignment after the procedure is unknown.
Another very common procedure is to have the leadpipe removed. One of the local repair shops to me had a standard practice for the 1980s Bachs of removing the leadpipe and cutting about 1/4" off of it and re inserting it. Immediate and drastic improvement in the way it felt for many individuals. However, now there are many options for leadpipes. It is beyond the scope of this answer, but you can search for replacing a Bach 42 leadpipe and find many opinions on the tromboneforum and other places. Gettin a good leadpipe that works with you and the mouthpiece you are using can make an unbelievable difference. More in my experience than what valve is being used to some extent.