Adding recipes is easy, changing the underlying mechanics is hard. Like with the robot thing, it would require making the robots easily craftable which is a no-no. idk if they're still craftable.
Not really? You can have separate uncraft recipes for taking apart the robots instead of making the main recipe reversible. This is also done as a common way to change what tools are needed to take an item apart.
In addition, don't forget that there's a distinction between broken robots and inactive robots. The former are generic items that can't be used directly, are dropped by robots when destroyed, and generally NEED an uncraft recipe to get parts out of since the item itself isn't craftable. The latter is the item for the actual deployable robot, and is what you get if you either craft it yourself or deactivate a robot that's been made friendly via control laptop, scrambler gun, or whatever. In this case you'd want to make its recipe (if craftable) not count as reversible and define a separate uncraft recipe.
You can also go the route of setting the recipe diffculty fairly low and then making the booklearn levels higher, and optionally add the SECRET flag to make sure that starting the game with those easy low-level skills doesn't mean you already start off knowing that recipe (as normally starting skills can let you start off knowing otherwise booklearn-only recipes). If you still want it autolearn you can even explicitly define what levels it autolearns at, or make it so it requires levels in multiple skills and not just the primary skill. Since this is DDA we're talking about, you could even lock it behind certain proficiencies like a robotics proficiency or whatever, as you can optionally set a recipe's proficiency requirement to be a "hard" one where you outright can't attempt a recipe without already having said proficiency.
All of this is real no-brainer easy JSON stuff to be honest, so "removing lenses from all these easy uncrafts was the lazier option" doesn't even really work as an excuse. There were multiple solutions to this problem and they picked the one that's the most detrimental to players.
I play both BN and DDA, DDA especially because as mentioned with the awesome Sky Islands mod. I definitely prefer BN for general use, although tbh I still feel it misses some of that danger from original Cataclysm, like you clear an area in BN or DDA and it's pretty much safe forever, although I guess DDA
One thing that if I recall both BN and DDA are missing is the old 0.B and earlier era style of dyamic spawns. I vaguely recall that in the old days, there was the current wandering horde setting where zeds can unload themselves and teleport around, while there was ALSO an option to choose between static and dynamic spawns. Static worked like the basic monster spawns do now, everything the game intends to spawn on the map already exists, while dynamic would set aside some amount of those spawns to instead show up later. 90% certain this was different from current wander spawns behavior, which is more along the lines of talking the existing monsters already determined to be in a given area and saying "okay these can now offload and be converted into a horde that will load back in and appear somewhere if enough noise while they're made in the reality bubble"
In addition, both versions pushed a lot of the most well-known and infamous special zombies from the 0.B era into being later-game evolutions. You used to have more day-one special zombies. I especially remember day-one zombie masters being an immediate source of "oh shit" moments when they appeared. I also vaguely recall they were like 90% of the reason why mansions used to be just pure weaponized death back in the day.
But in regards to the pipe rifle you would probably need a spring or something to strike the firing pin of the cartridge with enough force, but I think it's incredibly doubtful that you would be able to make it with just a screwdriver and hacksaw as per the recipe.
If we're talking realism about pipe rifles, they're most likely modeled after slam-fire shotguns. Those are basically just a pipe the right diameter to hold a shotshell, a pipe whose interior diameter will let the first pipe fit into it, and a nail or such set in place so pumping the two together makes the shell contact the pin. Caveat of course is if you went full realism you'd need to match the size of the pipe to the caliber used, and 12 gauge is the only one that's dead simple to match a fitting pipe to since 3/4 inch works about right for it.