In response to this question, please note to following points:
1- using plural and singular personal pronoun (we/I) for acts of Allah, is two different ways of assigning actions to Allah. Cases in which plural form has been used, indicate the role and functionalities of Allah's means such as malaikah in fulfilling actions, though in cases with singular form, not necessarily meant to deny their role, but it can be either immediate action of Allah or action that originates from Allah's will and is accomplished through his devices. Therefore, these ayahs implies that we have two types of assignments (attributions) to Allah: direct and indirect.
2- In this world, everything is based on the law of causality, and every phenomenon and every creature has a cause. Allah gives sustenance and this is done through causes. Allah is healer but this healing is done through causes. some part of these causes are natural and material and others are non-material. It is also the same in case of heaven and hell; Allah wants to reward or punish, they've done through malaikah. And also, Allah wants to intercede, does this through his intercessors (The holy prophet and his households). Thus intervening malaikah in rewarding or punishment, is similar to any other cause in the universe and this is obviously one of Allah's traditions.
3- Sometimes great people, when talking about themselves, are using plural pronouns, in order to show their power and greatness to their audiences, and to inform audiences that speaker and his speech are of great importance. For instance, in the event of creation and caliphate of Adam, Allah says: "we said"; because this happening is one of the most important issues in creation. So using all means of messaging is necessary to attract listener' mind to the sensitivity of the subject. Using plural pronouns is one of these means.
For further reading about the topic, please refer to following resources:
a. Ontology, Imam Khomeini Institute of Educational Research.
b. Monotheistic world-view, martyr Motahhari, discussion of the longitudinal and transversal system of the universe.
c. Allah justice, Martyr Motahhari.
4- Theology of the Quran, Allameh Misbah Yazdi, Discussing the Generalities of Allah actions.
5- Philosophical instructions, Allameh Misbah Yazdi, Discussing the causality principle