Mercury-Neptune isn't inherently deceitful. Neptune is always looking for some redeeming experience of unity and where Neptune is active in the chart shows where we can achieve this most naturally, but also where we are prone to see the opportunity for such an experience where it doesn't really exist. Mercury rationalises, problem-solves and communicates, and so a strong Mercury Neptune aspect - especially a 'hard' aspect such as a square - can reflect a tendency for these Mercurial functions to be affected by the Neptunian susceptibility to delusion. So we might be trying to be rational, to find sound solutions to problems, to communicate honestly and clearly, but the mind is clouded by the yearning for the sense of unity. In the case of communication, we might say what we think the other person needs to hear, or what we need them to hear so that they see us in a certain way - either way we are trying to move towards greater unity rather than greater separateness.
If you find that you have a tendency to be deceitful - or to be tempted to be deceitful - the close Mercury Neptune square is probably relevant, especially if you are able to take people in with a seductive style of communication. But the Scorpio emphasis could also be relevant for obvious reasons, and the Sagittarius emphasis could be relevant because this sign is very enthusiastic about sharing understanding - perhaps to the extent of feigning understanding so that it has something important to say. Note that Saturn squares the Sagittarius Jupiter from the 3rd house of communication, and that Saturn can bring an urge to overcompensate for a sense of inadequacy. Note also Chiron in the 9th.
There's nothing in the chart that says you are deceitful. But you might want to keep an eye on any urge to influence others with half truths or false statements, or to impress them using exaggerated or made-up stories. You would probably make a really good story teller, but its obviously best to be clear with yourself and others about which stories are true and which are fiction.
Best wishes
Miquar