Can the velocity curves of galaxies be calculated (and corresponding to what's observed), without reference to observational data?

I feel like you haven't understood the argument for the existence of dark matter arising from galactic rotation curves, because the question misses the point.

Read the section of the Wikipedia article on dark matter dealing with galactic rotation curves:

The arms of spiral galaxies rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the Solar System. From Kepler's Second Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed. Instead, the galaxy rotation curve remains flat as distance from the center increases.

If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

The argument for dark matter comes from Kepler's 2nd law:

A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

In the context of dark matter and the Milky Way, this becomes "A line joining a star in the outer galaxy and the galactic center sweeps out equal areas during equal intervals of time." Note this is independent of the mass of the star or the galactic center. The only restrictions are 1) the galactic center needs to be spherically symmetric, or the Shell Theorem does not hold, and 2) the mass of the star needs to be small relative to the mass of the galactic center (which is trivially obvious).

Therefore, you can increase the mass of the supermassive black hole in the Milky Way's center and it won't affect the argument for the existence of dark matter at all. The evidence for dark matter is independent of whether there is a supermassive black hole in the galactic center or what mass it is. It's sort of like how you don't need to take into account what your age is when calculating galactic rotation curves - even if you do, it doesn't affect the results.

By the way: the most massive black holes known are of order 10 billion solar masses, which is still comfortably less than the mass of the Milky Way (~ 1 trillion solar masses).