In fact “ne serait-ce que” is used when you want to give only one good reason and that it is implicit that there are many reasons.

For you second question, you do not need to place a “à” right before “cette mission”.
What will the mission bring back ? -> gold.

Hope that it helps.

Ne serait-ce que is used to introduce a narrowing down of the fact that’s been stated. Meaning can slightly vary according to context.

J’ai du mal à faire confiance aux étrangers. Ne serait-ce qu’à cause de ces histoires d’intrus.

Vous allez accepter, de toute façon. Ne serait-ce que pour l’or que vous rapportera cette mission.

In those sentences as ExteFrench says it is used to introduce the main reason not to trust strangers / to accept the job. ("… if only because…").

Ces gens vendraient leur âme pour avoir ne serait-ce qu’une bribe d’information sur l’incident.

Here it expresses a limitation, you could easily replace it with au moins ou au minimum (… "on at least one single piece of news.")

Ces gens vendraient leur âme pour avoir au minimum une bribe d’information sur l’incident.

Note that in a negative context, meaning differs slightly. It is synonym of même pas ("not even").

Il n’a rien voulu entendre, ne serait-ce que des excuses.

Ne serait-ce que is a "conditionnel présent". In very formal register and literature you might find the "subjonctif imparfait" ne fût-ce que. See the answers to this question for more.

A translation may be: "were it only…" which also uses the conditional and inversion.

J’ai du mal à faire confiance aux étrangers. Ne serait-ce qu’à cause de ces histoires d’intrus.

I find it hard to trust strangers. Were it only for this intruder business.

Vous allez accepter, de toute façon. Ne serait-ce que pour l’or que vous rapportera cette mission.

You’ll accept anyway. Were it only for the gold that this mission will bring you.

When used as an object, "but" is close in meaning, as in:

Ces gens vendraient leur âme pour avoir ne serait-ce qu’une bribe d’information sur l’incident.

These people would sell their soul to get but a morsel of information on the incident.