Simon Levermann wrote:

2. Libsodium exposes an API for random-number-generation. Should I use

this API for generating the salt when using argon2, or should I always use the openldap-builtin lutil_entropy?

Salts are not sensitive data, they're always stored in the clear anyway. The only property they're required to have is to be unique among a given population of users/passwords. lutil_entropy is preferable just for uniformity, but it's not really critical. You could use anything, as long as you're not using something like this https://www.xkcd.com/221/ .