Well, since Marcela is quite nasty towards Betty from day one, even before the latter had ever done anything other than being appointed Armando's assistant, it isn't hard to see why Betty isn't dying to empathize with her. Also, it's not like Betty seduced Armando, on the contrary he came onto her, and after all, even though her choice in that matter was obviously not right, she is not the one responsible towards his fiancee. Obviously her actively facilitating his unfaithfulness even before they got together was also wrong, but that's the point: Betty is desperate for some decent treatment, and Armando is one of the few people who (sort of) cares about her, whereas those who are most often victimized by his actions are people who are mean to her. So she idolizes him and chooses to turn a blind eye to all the nastier sides of his personality, and indulge his wishes. This is supposed to be a mistake, and she pays dearly. Also, of course a good single man wouldn't do, because they wouldn't want her (well, before her transformation anyway). If they did, Betty wouldn't have a reason to obsess over Armando in the first place. That's kind of the point.
Marcela is a very competent woman with a no-nonsense personality whose fiancé Armando constantly cheats on her with prettier and ditzier women, before being finally dumped for a homelier (in both phisical and personality wise) gal. Her actress was Isabella Santo Domingo, whom eventually wrote the satiric selfhelp-book-turned-tv-series Los Caballeros Las Prefieren Brutas, whose main theme is the downs of misunderstood feminism, and one of the hints it drops is that women should pretend to be dumber than they really are in order to get men to support them financially.
Hollywood Homely: Betty's actress is a very beautiful woman◊ in Real Life. However, the producers and makeup artists went to great lengths to characterize her as Betty, including hiring a specialist in colour to choose all the colours which were most unflattering to the actress. The "Ugly Squad" also has this; it was lampshaded when Betty, after meeting them, and learning their collective nickname, asks why Aura María, arguably as beautiful as the models who work for Eco Moda, is included in the group (her supposed sin is that she is a very short woman who wears clothes too slutty for an office).
Ho Yay: Between Armando and Mario, so much that Betty, overhearing their phone talk, thinks Armando is talking to a lover.
Lampshaded by Armando, who even tells Mario that if they keep being so close to each other, people will start to think that they are a pair ofHugos.
Ironically, the US version of the show would go down this road from day one, with the added twist that Betty refuses to take her boss's shit or listen to his excuses when he tries to avoid doing the right thing
Also Marcela, the informedly strong woman who still condones being constantly cheated on like an Extreme Doormat.