Hello Jeff,
Being a forum you are bound to get several opinions, but I will try to be a little more informative if I can. I have seen this exact situation several times before on the same species of tree - so I believe it is relatively a common situation.
Firstly, to review TreeTiger's comments. I don't agree there is decay per se - least not in a way that will effect structural stability. There is however, obviously bark necrosis ie the dying back of the bark. Yes, as suggested on 'the other forum' this could be as a result of mechanical damage eg strimmer/mower/dropping bricks on the area etc. but I don't believe this is the death knell for the tree.
As you can quite clearly see, there is a cambial regrowth/adventitious root that the tree is putting on in response to the damage (from the images I prefer that it is an adventitious root rather than its trying to occlude the wound - this would also match what I have seen in the field). Trees are self-optimising structures, so, if the tree is otherwise healthy and has a full photosynthetically active crown (can't see from the images), it will be able to compensate for the damage at the base for a good period of time - probably even outlasting how long you live at the property.
TreeTiger suggests 'probing' the area. I agree with this to a degree, but be careful. Please use a blunt object - or simply your finger, to press into what you called the heartwood to periodically check for excelloration in decay of this area. Don't use a sharp metal point as this can further damage the exposed wood and actually provide entrance points for fungal infection at a later point. Obviously the exact opposite of what you want to happen. As you say in your post though - the wood appears to be structurally solid, so it will continue to hold the tree up (in normal conditions) for a further period of time. This is combined with the fact that the tree is trying to assist itself by putting on the additional support root, so again reinforces my opinion that the tree is not near its immediate end.
As you have used the $$$ symbol in your post, you are possibly in the US (or Canada or Australia), so I am not in a position to comment further on your questions regarding the effects of different insects, although as a side note Black Ants ie small black common garden ants, are known to assist broadleaf trees by attacking smaller sap sucking insects eg aphids, that cause damage to leaves on trees - so perhaps killing your colony was not the best move? Pure conjecture on my part though, as I don't have enough information.
Regarding a fungi question - have you seen any fruiting bodies or other signs of fungal ingress? If not, I suggest waiting for your 'fungi season' to visually check for fungal fruiting bodies and attempt to make an ID. There are many guidebooks and of course the web - but you may also want to employ the services of a professional (or even post pictures on here!). Then when you are armed with further information ie is there really fungal ingress that would accelerate structural decay?, then you will be able to make a more informed decision about the tree's future. In the US, you may also be able to employ a 'Plant Health Care' specialist to treat fungi - but again, only if you know what the fungi is.
So, in the meantime don't rush to fell the tree as in my opinion there is little to be alarmed about. Get more information about what is going on with it, then you will be able to make a more sound decision.
All the best.