most of the times the battery light will come on, on the dash, or it will have a loud wurr to it... and the charge rate will drop to battery voltate eg. 12.6 or 12.8. if diodes are gone charge rate my be normal but untill you put lights ect on, will drop down to 13v or something... never realy seen one have any problems with supplying spark, cuz if it gets spark ecu is doing its job... how well known is your mechanic? as might be time to change... never seen an alternator do anything like what your sayen on any car.. weather it be ford, holden, nissan, subaru, mazada, ect..
i had the same symptom you have when i had my air flow meter disconnected, only as i had to get home, only with the airflow meter hooked up, it cut out and ran like shit.... my problem was that my engine was farked after it was majorly overheated but ran fine 70% of the time rb25det s2 though? dunno what yours is havent read your updates