You can't get a new lens for the Yashica A - it's lens is not interchangeable.
I would stick with window light and a reflector - only use the flood if you have no choice about working after dark, and even then, point it at a wall so that you have a broad, soft light source.
Expose the film at about half the rated film speed. Use whatever developer, time and temperature you are accustomed to, but test the combination first before you start doing serious work, and once you are satisfied, don't change anything.
Engage the subjects in conversation. The Yashica A is excellent for this kind of work because it is so very quiet, but it does tend to be a bit 'fiddley". I suggest putting it on a tripod so that you can frame and focus, and then not have to manipulate the camera as you interact with the subject. Shoot a full roll of film on each subject. Use a cable release (you will need a 'nipple' to attach a cable release to a Yashica A - most older camera stores will know what you are asking for)
While warming filters can help manage skin blemishes, I would avoid filters for this project. You don't want to lose any film speed (the minimum shutter speed on the Yashica A is 1/8 sec which is kinda slow for this kind of work).