Which ELBO I should use if I need to train the parameter from a real parameter and a simplex parameter?

1

Simplex parameter for a Dirichlet distribution.
Real parameter for mean and sigma of Gaussian distribution.

2

If there are no non-reparameterizable random variables (as it seems to be in your case) you can use Trace_ELBO. You can read more on this in SVI Part III: ELBO Gradient Estimators