The wall is wired with 8/3. and the white was not used, capped in the junction box. The breaker box has two-pole 30 amp breakers. I want to change the wire size in the junction box to feed the 18a 120v heater with 10/2, cap the red in the junction box, using the white/black/ground to the heater. The internal wiring of the heater uses 12ga. Can I leave the breaker box alone? Or do I need to change to 20 amp and blank out the red leg? The 8/3 in the wall will remain, and I plan to install the heater with 10/2 from the wall to the heater. I understand that 10/2 can use a 30a breaker. I think my question relates more to the breaker box. That would leave me with a two-pole breaker, only one of which us used. 8/3 to the junction box, 10/2 from the junction box to the heater, and 12ga internally in the heater rated 120v/2000w, no amperage specified.
As far as wire size, you're always allowed to use bigger wire than is required. Stay under 50A and that #8 will be fine.
As far as breakering that 18A heater at 30A, let's look at that. You must derate a heater by 125% of its nameplate rating. 18A x 125% = 22.5A. So you need at least a 22.5A breaker. You can round up. 30A looks okay to me, unless your AHJ insists on you using 25A.
Powering a 120V load off half a 30A breaker is fine. In fact I recommend using a 2-pole breaker for a 1-pole/30A load, because 2-pole 30A breakers are commonly used, and 1-pole 30A breakers are unlikely to have another use.