Who played 'allen' or 'arron' or whatever? The swarthy young guy?

You couldn't possibly mean "Cal" (Michael Emmet) who was fooling around with Liz in the swamp (and everywhere else) when her husband catches them together, could you? You know Liz - married to chubby store owner Dave. I used to love the scene when Dave leaves to make a grocery delivery and the other two are left alone together:

Liz: "You want somethin' Cal?"
Cal: "I sure do honey."

Of course, this being the 1950's, Liz and Cal get what they have coming by the giant leeches for their, um, indiscretion.

This film is one of the great ones. They don't make them like this anymore, and I sincerely wish that they did.

I think this post is about the goateed black-haired young man with the necklace.
He appears at 3 min 40 seconds into the movie and asks "Lem Sawyer"(played by George Cisar in the movie) about the thing he shot in the swamp.
Later at 42:34 in the movie he declares that there are no gators in the swamp and that "nothing scares gators". That man is identified as "Evan" at 42:34 by Steve.

The goateed man identified as "Evan" is also milling around in the background at 52 minutes in (so it eliminates him as being "Mike"). He gets another face shot at 1 hour in as the scuba team is returning to the shore.

Of the characters (by process of elimination) the uncredited "Evan" could be Guy Buccola and Ross Sturlin (who played the Giant Leeches) or given a bit of trivia I found, it could likely be Actor Ed Nelson (uncredited).
http://www.imdb.com/name/nm0625343/?ref_=fn_al_nm_1

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There is a man that dies in the movie at 9:52 and he is identified as "Lem Sawyer". Which clears that character off the list. The blonde woman in the horizontally striped shirt is Liz Walker (played by Yvette Vickers).

The adulterer pair is likewise identified as Cal Moulton (played by Michael Emmet) and Liz Walker (played by Yvette Vickers). Both of them die in the underwater Giant Leech cave. The angry husband that kills himself later is Dave Walker (played by Bruno VeSota).

The hero and annoying environmentalism (who also increases the number of unnecessary human deaths by refusing to dynamite or drain the lake) is Steve Benton (played by Ken Clark). His girlfriend is Nan Greyson (played by Jan Shepard).
BIT OF TRIVIA -- Ken Clark also played the beefy character of "Stewpot" in the South Pacific musical movie and even ran around in red speedos while flexing.
http://neptsdepths.blogspot.com/2012/12/im-really-in-stew-over-this.ht ml (NSFW Website link)

There is a character called "Norton" that appears as the Sheriff Kovis (Gene Roth) talks to a fellow sitting on a chair in the police office. He is uncredited.
Doc Greyson (played by Tyler McVey) is talked to by the hero the Doc Greyson is identified as the father of his girlfriend.

The two older men in the boat who are looking for a reward are identified as Old Sam Peters (Joseph Hamilton) and Porky Reed (played by Dan White).

Steve Benton identifies a "Mike" as also having scuba training.
At 51:13 though the young black-haired man in swim trunks and scuba gear taps Steven Benton's scuba tank, goofs up his line, and asks "Comfortable? Are you sure you don't want me to go down there Mike?". The shorter black-haired man is identified as "Mike" (played by Walter Kelley) in the IMDB website though.

And that is where the name listings run out.
http://horrornews.net/1242/attack-of-the-giant-leeches-1959/
Playing the leeches were two basketball players, Guy Buccola and Ross Sturlin, who were given the job on two criteria: They were tall enough, and they agreed to make the costumes themselves. Actor Ed Nelson was supposed to make the leech costumes but he dropped out at the last minute.

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'Mummified' Former Playmate Swerved Between Recluse and Storyteller
May 4, 2011
Those closest to former actress and Playboy playmate Yvette Vickers say she led a dual life: one as a private recluse hidden away in her Los Angeles neighborhood, and another as a vivid storyteller who charmed audiences at film festivals and kept in touch with her fans years after her star had faded.

Now police believe the badly decomposing body found in Vickers' Los Angeles home, although not yet identified, is indeed the 82-year-old woman best known for her role in "Attack of the 50 Foot Woman."

The coroner's office has yet to determine how long the body lay in the Vickers home

Over time, Vickers' friends and neighbors say, she descended into paranoia and hoarding, her home eventually filled with black mold.

In her heyday, she had appeared in several films, notably "Hud" and "Attack of the Giant Leeches." Her first movie was "Sunset Boulevard," a 1950 film about a former movie star living as a recluse in a ramshackle home. For just a few seconds on screen, Vickers played the part of a laughing girl.

"She was a wonderful lady and I'm so sorry and sad because I wished I could have helped her," Margaret Netcell said, her voice choked with sorrow. "I just can't imagine dying like that."

Doing a GOOGLE image search, the uncredited character (he had a speaking role) of "Even" resembles actor Ed Nelson more than he resembles actor Ross Sturlin.

Attack of the Giant Leeches (1959) AKA "Demons of the Swamp" (British Title)
https://yooniqimages.com/images/detail/102408862/attack-of-the-giant-l eeches-aka-demons-of-the-swamp-unidentified-attack-of-the-giant-leeche s-us-1959-aka-demons-of-the-swamp-british-title-unidentified

Ed Nelson (I) (1928–2014)
http://www.imdb.com/name/nm0625343/?ref_=fn_al_nm_1
Age in 1959 = 31 years old

Ross Sturlin (1931–2002)
http://www.imdb.com/name/nm0836392/
Age in 1959 = 28 years old

So they'd both be around the right ages.
Small correction though. If the release date was wrong, then the actors would be a bit younger.
This website (where I got the Ed Nelson trivia) lists the date of the movie as 1957 though the website lists it as 1959.
http://horrornews.net/1242/attack-of-the-giant-leeches-1959/
Attack Of The Giant Leeches (1957)

Curiously the title cards have a bit of confusion too.
At 1 minute 52 seconds into the movie, the title cards list the release date as
MCMLVIII = 1958 by Balboa Productions.
1959 in Roman Numerals = MCMLIX
So either they changed the release date per the distribution changes or this movie is a tad older than is commonly known.

Oh, one more possibility of the guy who played "Even" might be a fellow that appeared in a few Roger Corman movies.

DON SULLIVAN
http://www.imdb.com/name/nm0838042/
http://roberthood.net/blog/wp-content/uploads/2011/12/don-sullivan.jpg

Unknown man called "Evan" from "Attack of the Giant Leeches"
https://yooniqimages.com/images/detail/102363623/attack-of-the-giant-l eeches-aka-demons-of-the-swamp-unidentified-dan-white

Here is a quick image show of DON SULLIVAN ("I sing whenever I sing..." comparing facial features with the uncredited black-haired male actor with a goatee from "Attack of the Giant Leeches". This image shows Don Sullivan with a similar image taken from the set of "Attack of the Giant Leeches", "The Giant Gila Monster", and "The Rebel Set".
http://i60.tinypic.com/2z844cn.jpg

He is an extremely close match to the uncredited "Evan" character from "Attack of the Giant Leeches" and statistically-speaking there is a pretty good chance that Roger Corman employed Don Sullivan for this role, but somehow did not credit him.

Don Sullivan is not listed as being deceased, so if anyone can do the legwork and interview him, this role could be confirmed. He is noted to have been 18 years-old while appearing in an episode of The Adventures of Wild Bill Hickock that aired in late 1956. Which would make his birth year 1938 and his age in 2014 equal to 76 years-old.
Ray Kellogg died in 1976 so he cannot be interviewed.