Bad news! Usually, if the offending particles are simply dust and dirt that live on the surface of negatives and somehow become embedded are removable by the detergent action and emulsion softening of Photo-Flo. May I suggest, as an experiment on one not-so-important negative, you re-do the procedure but with an extended time- 5 to 10 minutes is the solution and more aggressive rubbing.

As for "archival" envelopes and sleeves- I have had some issues. Some glassine envelopes were labelled as archival that were not. I don; know whether it was the material itself of the glue or adhesive used to construct the envelopes and/or the storage conditions. There was severe adhesions to the negatives, however, I was able to dislodge the negatives with the soaking procedure but there was another permanent damage other than spots, rather Moiré-like patterns. Some of this was somewhat remedied by printing the negatives WET in a glassless enlarger carrier with a cold-light source.

At one time, I discussed this with a film archivist who mentioned that residual chemicals in the film can act as a solvent and cause this kind of adhesion and subsequent damage. High relative humidity accelerates the reaction.

I get into this stuff because at my studio we do quite a volume of photo-restoration for museums and archives that don't have in-house technicians who are familiar with photographic restoration as opposed to the restoration of paintings, etc. Now and again we get the damaged and dirty film, glass plates, and lantern slides where there is interest in the historical content of the image and restoration is need just to clarify the details. Many of the artifacts are too fragile or will simply disolve if soaked. Others are beyond cleaning so out come the airbrush and the cloning tool on the computer. Not fun!

I have found a solution- my lovely wife loves cloning out spots. She is a fastidious housekeeper, organizer, etc. You can perform open-heart surgery in our kitchen!