1) I’d say direct object, but as a native, my competences in grammar are not so formal. Direct because if you ask “who?”, then the answer is nous.

2)Dieu les garde qu’il le soit

3)Yes, it’s a believer way to say : please, have god spare us …..

4)Incredibly oldschool, france is a country of non believers (or at least soft believers, for most of them), so i’d advise finding another more rational form, such as “Faites qu’elle n’oublie pas” (still religious based, but more widely used).

1. This nous is not an indirect object

2. Put differently, with a 3SG or a 3PL, the sentence becomes:

   Dieu le/les garde qu’il le soit !

3. Yes, the que-clause is being wished against. We would like to be spared its happening.

4. In French, there is a classical alternation between que-clause and an infinitive clause in this context

   Dieu la garde de l’oublier !

   This alternation appears in many contexts:

   Je veux qu’il vienne <=> Je veux venir
   J’ai demandé à Paul que sa femme vienne à 10h <=> J’ai demandé à Paul de venir à 10h

In the original construction, nous is a direct object. This might appear to conflict with the presence of a que-clause which seems to occupy this function. But in fact, it looks like an ellipsis of de:

Dieu nous garde (de ce) qu’il le soit.

Strangely, de-headed que-clause (as well as à-headed ones) always feature a ce between the preposition and the que.