I believe the Empu Gandring and Ken Arok legend was picked from "Serat Pararaton" (= Para Ratu, Cronicle of the Kings), which was written nearly 400 years after the actual incident happened ( Ken Arok / Singasari reign). It is, certainly, hard to verify, even at the time when the book was written. Some experts even interpret this "Empu Gandring" story as a symbolic languages (or gurindam, as Mr. Purwacarita define) depicting the "magnificent" struggle for power in Singasari between the families. But IF only this legend was TRUE, those who knew the legend would understood that the keris had not been finished by Mpu Gandring, which then caused Ken Arok, who've been waiting for a long time, to became very angry, grabbed the keris, and killed the empu himself. It was, actually, the blessing ceremony which had not been done. Mpu Gandring himself had warn Arok that the keris was fiery and full of anger when he came to took the keris. The curse which was spelled by Gandring while he was dying, would be the "blessing ceremony" then.
Making keris is an art, technically and spiritually, and can not being done in rush and anxious circumstance. The fact that Ken Arok was a well-known leader of robbers, would also influenced the "spiritual circumstance" of Mpu Gandring. Imagine this : How would you fell if Osama bin Laden commisioned a keris to you, if you an empu ? Your feeling or "spiritual circumstance" then would changed considerably and being easily absorbed to the keris you make, and nothing you can do about it. You have at least two options then : Stop working, which would only make the keris-making process delayed while the feeling still "haunt" you. Not to mention Mr "O" who may get angry for waiting too long. Or refuse the commision, which may cause Mr "O" send you a suicide bomber

Same situation might also apply to Gandring, and thus, it is easily understood then, why his keris so fiery and full of anger, even before Gandring cursed it.
I do agree with Mr. Purwacarita, it is almost impossible for empu, especially Master Empu, to "cast" bad intention in his keris. It was a "mishap" that made a bad keris. This might be technically or spiritually when the empu lost his concentration. Any disturbances which make the master empu angry, in doubt, anxious, annoyed, sad or any "bad feeling" would easily seep into the keris. Just like a composer when he compose a song. While a composer use a song as a media to express his idea, feeling, or wishes and a painter use the brush's strokes and colors, an empu choose the iron, pamor and others on a keris to serve this purpose. You may choose a song as "your song", or a painting which "describe" your feeling. You may also choose a keris to reflect your wishes, 'ideal character/personality' you wish to achive, your ideal 'way of life' or ambitions you wish to achieve(= 'Da Flag'). Then, the keris may serves as a 'remembrance', as every time you see your keris, you will remember of your wishes and gives you a "power/motivation" to achive it.(=Da Nationalism). Selling this kind of keris then would be felt very humiliating to the owners. Sometimes, on a very good pusaka, "the power" is so intense, that any "keris-illiterate" who see it would also overwhelmed by this kind of "unseen/spoken power". Just like any listener would be overwhelmed by a good song likes, said, Bohemian Rhapsody. This explanation, however, only explain the esoteric things from exoteric point of view. (perhaps an "exo-esoteric" explanation

) The real esoteric would also involve the empu's prayer/blessing.
Then, it is very easy to understand that the keris for a soldier would not match the wishes of a farmer. A merchant should not wear a keris intended for a king. Why ? Because his costumers want to be king also. Two king in one shop would only ended up in a war

Mahar-ed or bought, it will not make a lot of differences in this kind of "esoteric". It may also easily understood that "the old fashioned" Javanese would very reluctant to show his pusaka keris to a stranger, which may also show his ambitions, which is considered as unpolite in Javanese culture. It is also speculated that many good pusaka's ganja was changed on purpose with "ganja wulung" (=black ganja, that is, without pamor) to hide the ambition of the wearer/owner. (Yes, it is very possible to do an "educated guessing" on keris' pamor and dapur, only by looking the ganja on a sheathed keris. An expert may reach 80-90% accuracy. With ganja wulung, only dapur could be predicted, certainly).
Making keris technically, also took a lot of care and concentration. So, if you had a technically perfect keris, it would be almost always guarantee you a good concentration of the maker, which may also connected to "good spiritual circumstances". It may also always guarantee you profits, if you bought it on a fair price
"Yoni" jargon to describe the unseen "esoteric" things, IMHO, would be a "recent" invention. "Serat Centhini" which was written in 19 cent. did not use this terminology (it describe the meaning of ricikan and dapur though). It was said that Mr. BPH Sumodiningrat, a Surakarta's court prince and also a respected keris expert, who introduced this terminology. (Unfortunately, Mr. BPH Sumodiningrat had passed away) He did not, as I recall (remember please, I have a short memories), explain the Lingga. IMHO, the "yoni" term would resembles the "Yin" side in Taoism( or Daoism, a more approriate spelling), while Lingga is the "Yang" side, the "seen" quality, and thus, the blade itself or exoteric things. Today in Java, Yoni terminology usually use on every "unseen" or "spiritual/spirit" qualities which reside on something, not limited to the female genital symbol where it was originated.
I do aware that some spirit/energy things may reside on kerises. I also aware that some of them reside on my collections. While I'm aware of their presence, once again, it is not my main concern/consideration on selecting kerises. Until today, we live happily and peacefully

Serat Centhini warned "be careful on selecting keris. The pamor/iron may bring you a good luck or a bad luck." It did not warn about the spirit or even wrote about it, as I recall.
One of the elders warn me :"Just like other traditional arts like wayang (shadow puppet) or Gamelan, the keris standards, as a classic Javanese art, had been set. Anyone who seeks other standards would only get lost." Perhaps, it is only a Javanese "chauvinistic" point of view
Yes, Tim. It is sad to say that almost all about money. It is a fact of live

Money may not buy everything, it only able to buy 99% things in this world