I'll admit I was wrong with the case that should be used, as 'like' clearly is a preposition in this usage.

However, I do not see how 'myself' is a wrong word to use. As I think about it, the speaker of the sentence obviously counts himself amongst the number of gardeners, especially those who are serious (and all those who are Scotsmen) in the world, and could therefore use the reflexive as the object of this proposition. If we were to combine the speaker and his wife, would "Serious gardeners like us always use organic fertilizer," be any different from "Serious gardeners like ourselves always use organic fertilizer."?

I would argue that the use of either the reflexive or the objective pronoun would influence the tone of the sentence, and its connotative meaning.

The pronoun "me" would not be grammatically appropriate in this instance, as its referent would in a subjective position which would demand the subjective form. "Myself" may work because its referent has already been mentioned in discourse, under the umbrella of gardeners.

My-, hi-, herself and the like are not necessarily the reflexive pronoun. They may also fall into the intensive pronouns, which may be used in any part of a sentence, either objective or subjective. I would in this case take issue with the use of "myself" in the sentence "I fixed the car myself," to be adverbial. It is intensive, and as a pronoun, works as an appositive to the subject "I".

I've heard of protons and neutrons as being massive, i.e. the baryons, but not electrons. The baryons are named because they are the major contributors of weight (GreeK: barys) to regular matter, and as such are massive, having a lot of mass.

I think that the indefinite article is completely valid.

I think that's perfectly fine. People just wind up making the wrong assumptions of words by only hearing them in certain contexts. My father once had issue with me using the word 'titular' in reference to Hamlet in the eponymous play, thinking that titular referred to the title of a person, not a work. Your friend just has assumptions that fetch goes with dogs and that dogs are less than people, therefore fetch shouldn't apply to people.