"May I have every one?"
Translation:Ga i bob un?
You are right, i (preposition) does cause soft mutation of the following word if it can.
This i means 'I', however. In Ga i...? (May I...?), i (I) is the subject of the verb ga but is not itself causing the mutation. pob un is mutated to bob un because it it the object of the verb ga:
- Ga i...? - May I...?
- Ga i brynu cinio? - May I buy lunch?
- Gaiff Siân ddod gyda ni? - May Siân come with us?
Any 'short-form' conjugated verb will cause mutation of its object, if it has one.