If two regular transvaginal us on two different days of my cycle suggest "possible polyp" . how likely is it that a polyp will be confirmed on my saline us coming up?

Saline sono: more accurate for polyps in general. So could go either way.

Somewhat likely: If the u/s were conclusive they wouldn't order the saline sonohysterography. A lot of things can look like polyps on non-saline u/s that really are not polyps. There is a descent chance that there will not be a polyp there on sonohysterography. Best wishes!