横 (heng2) is its own character, as is 竖 (shu4). 一 and 丨are never pronounced as heng2 and shu4, but the shapes are referred to in context of discussing calligraphy, stroke order, etc.
Here's an analogy with the letter "o". Is it a circle? Yes. Do we ever read it as "circle"? No. But just like "o" is a circle-shaped letter, 一 is a 横-shaped character.
In terms of which words are used in practice and are understandable, either check out a dictionary or just ask a Chinese speaker how to pronounce the character. The first answer you find is likely to be the most common and useful. The secondary pronunciations, names of strokes/shapes, and even some of the characters themselves are not common in everyday language, and will only pop up in the context of calligraphy, linguistics, historical research, or some areas of law and science.