Is the variation in the spelling of the agent-noun suffix -er/-or related to any phonetic difference? [duplicate]

Question

This excerpt in a book on English suffixes piqued my interest. Is there any phonetic correlate between the way the nouns below are pronounced and their spelling? I know execut-or/-er admits two stress patterns, but perhaps there are others as well. Since all these spelling conventions seem to vary somewhat between dialects and speakers I was wondering if it was the cause or effect of a difference in speech.

abstractor (= -er), connector (= -er), constructor (= -er), contradictor (+ -er), detector (= -er), detractor (= -er), impactor (= -er), obstructor (= -er), protector (= -er), selector (= -er) tormentor (= -er), adjustor (= -er), protestor (= -er), executor (= -er)

Answer 1

A conclusion on the basis of this short list is obtained from the SOED (The Shorter Oxford English Dictionary); it can be summarized as follows.

In most cases the suffix is "-or" (exceptions: "abstracter, adjuster") and the variant in "-er" does not exist (or must be extremely rare) or is rare. In rare cases (1) both suffixes are in use ("protester, protestor") In rare cases only (2) is there a difference in meaning.

The suffixes "-or" and "-er" are not stress imposing and therefore stress is preserved in all cases, except as concerns "executor", where the pronunciation \ɪg 'zek jʊt ə\ (main form) is an exception to be contrasted to \'eks ɪ kju:t ə\, a pronunciation reserved for the less important word "executor" as meaning "performer" (stress according to the usual stress assignment as required by the suffix).

Difference

• abstracter a person who abstracts; a person who makes an abstract or abstraction.
  abstractor an abstracter; spec. (now Hist.) as a title or a grade of clerks in the British Civil Service.

• connecter see CONNECTOR
  connector Also (esp. of a person) -er

No difference

• constructer obsolete
  constructor

• contradicter (does not exist¹ (user LPH))
  constructor

• detecter see DETECTOR
  detector Also (now rare) -er

• detracter see DETRACTOR
  detractor Also (now rare) -er

• impacter (does not exist (user LPH))
  impactor

• obstructer (does not exist (user LPH))
  obstructor

• protecter (does not exist (user LPH))
  protector

• selecter (does not exist (user LPH))
  selector

• tormenter (does not exist (user LPH))
  tormentor

• adjuster
  adjustor (does not exist (user LPH))

• executer see EXECUTOR
  executor

• protester (more important form, ngram)²
  protestor = PROTESTER

¹ _{i.e. does nor appear as an entry in the SOED}
² _{not in the SOED, added information}