Ha... just realised I didn't actually answer your questions!
If you want a response from Walter, maybe try emailing him (I think he is the admin contact for this board and for the Demodex Solutions page), or maybe post in the "Acne and Rosacea" forum (that seems to be where all the action is).
As for the metronidazole treating the blepharitis, I have read some abstracts for papers on the web that were both for and against its use in treating demodex...
Some report favourable treatment:
http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract
Another study says that demodex can survive in what seems to me to be a concentration of pure antibiotic (1mg/ml):
http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract
Now, assuming it does work, the mites are either living around the base of your eyelashes, or within the meibomian (oil) glands on your eyelids. According to what I have read on this site, the reason that you may need to treat for a long time (3 months or more) is because it is hard to get the medicine to the location of the mite (within the oil glands). So what you usually do is kill them off gradually, because you can only kill off those that surface, slowly depleting the population of the little buggers...
Before I start up using the metronidazole again, in what may be a long treatment process, I want to check under the microscope that the treatment is suitable. You may want to do the same.
I'm assuming your GP or ophthalmologist prescribed the tetracycline for your condition, in which case I wouldn't change your dosage without talking to him/her first.
As I understand it the tetracycline is intended to thin the oils produced by your meibomian glands, thus helping your eyes produce a normal tear layer and hopefully flushing out any infection. The other theory, that I think I have read here is that the antibiotic effect fights the infection caused by the mites (which carry bacteria), thus limiting some of the physical symptoms (on the face this can be acne, on the eyelids, maybe swelling or styes?) but not removing the cause (the mites).
Whilst the people on this board offer some great advice, I think you should always check with your doctor before changing medications or starting new ones...
Let us know how you go.
Lindsay.