Is it allowed for a women to smile occasionally and have an occasional chuckle during the ‏ناصر الشرعية, if this is part of how she normally talks, or must she suppress this? When I am nervous, I tend to smile more and chuckle more. I don’t intend this in a flirty way.

What is forbidden is laughing with him and making your speech soft in a way that invites him, because Allah Jalla wa ʿAlā (the Majestic and Exalted) says, the meaning of which is: do not be soft in speech, lest the one in whose heart is a disease should covet. Speak to him normally, with a mahram present. Be yourself and if you usually smile or so, that is not a problem as you said it is not done in a flirtatious way.