Learn Spanish (Latin America): Con relación al desarrollo, el concejal se lo explicó a la prensa. - In relation to the development, the councilor explained it to the press.

Yes, they are practically interchangeable for introducing a topic. Con relación a (or en relación con) sounds slightly more formal or bureaucratic, which fits perfectly here since a councilor (el concejal) is speaking to the press. En cuanto a is just as correct but is also used frequently in everyday casual conversation.

What exactly do se and lo stand for in se lo explicó?

Lo means "it" and is the direct object referring back to the topic just introduced: the development (el desarrollo). Se is the indirect object meaning "to the press." Normally, "to the press" would use the pronoun le, but Spanish changes le lo to se lo so it flows better when spoken.

If the sentence explicitly says a la prensa (to the press), why do we still need the indirect object pronoun se?

This is a classic Spanish feature called indirect object redundancy. Even when you state the recipient clearly at the end of the sentence (a la prensa), it is standard and often required to include the indirect object pronoun (here, se standing in for le) before the conjugated verb. The a la prensa phrase is just there to clarify who the se refers to.