Fact#1: Secrets are fun!
Fact#2: Secrets are difficult to keep hidden!
I have had an interesting RP history with you, Leo. So what I say is meant as positive advice:
I can understand the appeal in the concept of Stillwater. Having an apparently legitimate business that pays it's taxes, offers respect to the clergy, attends charity event to keep a friendly face to the public, plans business meetings with your peers; both friends and competitors...
All the while there is a shadow side to it. Dark deals are made hidden from the public eye, and your thugs make nasty things happen that make your operation that much smoother. When your competitor meets his brutal end you sob in front of the cameras and offer your condoleances to his family right before you devour what remains of his disintegrating business empire.
Sounds like a ton of fun!
But... just as in IRL, a secret only survives if you hide it very carefully. If you start keeping a secret you must be prepared that it might come to the light eventually. When that time comes you must either have a plan for damage control, or just go with it and admit your true loyalties.
Even without looking at public profiles and forum sigs;
In-character by now many players have seen Leopold say things, do things, act in certain ways that incriminate him. These could be bystanders, participants, victims, former employees... anyone! After this happens the secret will no longer be ironclad and people will start whispering. Enough whispers will create a rumor. Given enough time and no satisfactory counter, the rumor will be accepted as "truth". This has now gone on for so long that not a lot of people, or at least that I interact with RP-wise, have any doubt left that Leo and Stillwater are Cartel affiliated.
Whatever damage was done by IC interactions cannot be reversed (and they shouldn't). However, if you insist on maintaining the legitimate businessman routine I would:
1. Change your public profile, and that of the corp to make it look completely legit. I agree that putting things between brackets looks stupid. Sadly most people (including me) disagree that the public profiles are OOC information, and that isn't going to change any time soon. That doesn't mean that you can't tell your story at all. Instead I would put a link at the bottom of the profiles that directs to a wiki-article on the Evelopedia where you explain about the Angel Cartel Affiliation. That way only those with the required attention span to appreciate intrigue will read about it, and the rest is none the wiser.
2. Be more discreet in player interactions and avoid making Leopold getting his own hands dirty as often. Being a crimelord involves using mediators and henchmen outside your corporation so that they can take the blame "they were mercenaries acting against their contract".. etc.
I hope that this made sense