This goes back 40 years in RV construction. This keeps the upper and lower skin from vibrating and flexing right at the end of the stiffener (flange riveted against skin). Cracks form. The sealant keeps it stable.
Long ago it was discovered very careful work you can get the stiffener flange closer to the radius of the trailing edge. Closer means right where radius starts. The issue of cracks forming right where flange stops typically happens when people leave a long space from end of stiffener to trailing edge radius, leaving skin unsupported. The goop was a fix to large gaps of unsupported skin.
Silicon can be used if you prime the surface. Silicon is acidic and will etch aluminum and this can lead to corrosion. As long as do good surface prep (etch, alodine) and prime silicon is good.
Yes pro seal will work. I do not use any glob of any goop, I just make sure the stiffener is right up to the radius. You have to relive the stiffener free flange so it does not touch the opposite side of skin. Allow some room for movement.
Prime and use RTV or use Pro seal. Do not recommend chewing gum. Ha ha.