I feel I may be able to help a little here, because I have suffered from such a nickel allergy.
However, before I develop that subject, I do wish to express my immense thanks to the superb Ernie Balls Customer Service, and the guys at Strings and Things for their Professional and Rapid Assistance. In particular, I would thank the excellent JM Quinn, AJ Braman and Nick Clubb, though everyone seems to be particularly good at their job at the Factory!
There is no doubt that at EBMM there is conspiracy inspired at the top, where people and the attention they give to their work make the entire difference, that keeps Great Musical Instrument Design & Manufacturing still available to a desiring public and EBMM the Creme De La Creme.
Nickle Allergy is quite a common occurrence, and in reality, a form of Dermatitis.
For the female sex, ear rings with Nickle content usually cause redness and itching around the ears, and are often the earliest indicator of sensitivity.
For the male sex, watch backs (even those they state on them that they are made from Stainless Steel usually have a small degree of Nickle in fact), and so the inside of the wrist is usually the earliest indicator of sensitivity.
I do not present myself as an expert on this subject, but rather as an individual with profound empirical experience over a great many decades, covering personal use, product and customer experience, and manufacturing issues regarding this matter of concern, which involves very, very few Players, in reality.
The good news I have to give you, is that often, when the problem is experienced it is not caused simply by one thing in my personal view (thought it can wrongly appear for that to be the case), and that there are a number of interrelated issues, which are in fact, usually involved in creating the actual problem for an individual.
Because the Ends of Players Fingers, build a thick wad of dead-ish skin tissue that is highly protective, I am possessed of the belief that for the most part and most people, the absolute minimal contact involved with Strings and Frets, on such a small portion of 'thickly desensitised skin', are unlikely to be the REAL reason, underlying any problem that might occur.
Usually Cleaning Products such as Soap and Detergents etc, are often also involved but completely overlooked, as are Toiletry Products, After Shaves and Perfumes. Rubber, Metal and Garden Plants and Chemicals can also be detrimentally involved in such a complaint, and this is also often overlooked, the environment in which exposure and the problem occurs.
Excessive, repeated hand washing, promoting continual contact, often with detergent chemicals, removing the skins natural oils which build up only after a considerable hours may be a factor involved for certain people
To me, it is no coincidence that in hot and humid weather, the sweaty area underneath a watch back or behind an ear, in contact with the skin is where things become a problem and first noticed. For this reason many watch makers place a clear plastic barrier on the watch back (which should be left on the watch but is usually removed) and some even supply watches with a hinged plastic back as protection. I owned such a watch and for a time it was the only one I could wear at all.
On a practical point then, simply removing the moisture of sweat regularly, keeping small towel handy for ones fingertips and hands can thus assist a Player whose hands might well be perspiring during a busy Session. Furthermore, the type of more natural Neck Finish found on so very many EBMM Instruments, are less likely than those found most available alternative Instruments, some of which definitely cause excessive sweat in the first place.
So I think you are far better off with an EBMM Instrument, than most others, in this important respect.
Another point is, it is my observation that there is a degree of what I would broadly call a 'super sensitised nervous system' involved in the occurrence of this complaint, and possibly a psychological element, or a psychosomatic component. Please appreciate, the allergy is not caused by such a condition, but it is 'greatly increased and exacerbated' by anxiety, nervousness and stress, elsewhere in the individuals life.
Eliminating far more common sources of aggravation, for example using a neutral Ph soap etc, and dealing with the underlying causes of negative stress in one's life, in my humble opinion may prove to be far more productive in controlling and overcoming the effects of such an allergy, and much more helpful than any great concern regarding level of the Nickle Content in Guitar and Bass Strings.
And I write as someone who could not even wear a watch on his wrist for some time because of sensitivity to Nickle content.
In my experience, dealing with soaps, overcoming issues that cause great stress (Stress can creep up on us unawares, and build over time) and carefully controlling particular areas where sweat is focussed, so that things do not get out of hand in the first place (cleaning the Instrument regularly and properly plays a part here), and giving yourself the needed time to completely get over any occurrence of allergy experience, should meant that finally, in the end worrying about Strings and Frets, should not be an issue at all for you in any way shape or form.
It is difficult to imagine, any more minimal or desensitised a point of contact than a String Fret or Finger Tip of deadened Skin.
If you want to deal with this issue, look at the problem in as broad a way as possible, carefully observe the conditions, cause and effects involved and eventually, you will be able to bring things into control to a point where String Materials and Frets are a non issue.
For it is probably the case that there are in fact a 'number of other elements' that you are also allergic to, but completely unaware of as yet, and tracing these and dealing with them will be important in completely eliminating the problem.
This is my considered opinion with many, many, many decades of observation, experience and dealing with Players Requirements, Musical Instruments and Manufacturing Industry.
Quote: "I have a severe nickel allergy which started around 6 years ago, and I've been playing for 35 years."
This confirms the truth of all that I have written above.
If the Strings or Frets were the REAL problem, (I concede that they may appear to be part of the problem for a short while) then the difficulty would have obviously arisen long, long ago.
You can't play for 29 years, and it 'suddenly appear' and for the Strings and Frets to be the real culprit. In my experience, there are other factors which when taken fully into account and dealt with, will restore the previous healthy experience with Guitars and Basses for the individual involved.
That is an unassailable axiomatic truth that is impossible to get around.
Best Wishes to all!
P