Angels

Some people say it is because Allah had created the jinn first, and the angels had seen the jinn fighting so knew that a new species with free will would also fight.

Some say that there were other humanoids (nasnas), and they had fought, and so they deduced from this that a new species with free will would also fight.

These two views are reinforced by a narration from Imam al-Sadiq (A) in which he says that the angels could not have known that humans would cause bloodshed if they hadn't seen the precedent of earlier creatures on earth.

Some simply say that they understood this from looking at human nature, or by looking at the nature of life on earth and the challenges of living as an earth-based being (for instance, competition for food, water, reproduction, and land).

Since the earth (dunya) is considered the basest or lowest level of creation, vis-a-vis the heavenly realms, perhaps it also didn't make sense to instill Allah's vicegerency in an earthling. 

Maybe there are other reasons too which relate to the nature of the angels and their existence outside of the earthly realm which would give them access to knowledge or a perspective that human beings generally lack. 

It is ok to address a secondary cause (whether it be Imam 'Ali, an employer, a relative, etc) to request specific assistance as long as one agrees that Allah is the primary cause and these are just secondary means.

There is theoretically no problem with addressing the angels as well. However the Qur'an says that the angels fulfil God's command as God gives it to them and therefore this suggests they do not usually act according to the wishes or preferences of human beings. For instance, if an angel assigned to deliver sustenance, it will deliver what it was told to and not what the person wants. So for instance if I just casually say "Ya Jibra'il", it might not have the same effect. Of course God knows best. 

The Qur'an says that the angels do not disobey God and do as they are commanded (Qur'an 66:6). It is also narrated from Imam 'Ali (A) that the angels do not disobey or tire. From this, we can glean that the angels do not disobey Allah.

Possibly, angels may have the inherently capability to disobey but simply choose not to do so, because they have no reason to do it and no inclination to do it.

This is often understood as being because angels have intellect but no animal desires, unlike human beings, which have both intellect and animals desires, and usually it is either our bodily urges or related traits (such as anger) which cause us to do wrong.

The angels also have a greater awareness of higher realities (such as things pertaining to Allah) as well as the ugliness of some wrong acts, so perhaps this also may contribute to not wishing to do wrong things, unlike people, who are sometimes blind to these things. 

For this reason, stories like that of Fotros can be considered questionable.

God knows best!

I have not come across the idea that the angel Mika'il keeps the devil out of heaven in mainstream Shi'i or Sunni texts.

Also, the Qur'an says that God Himself threw the Shaytan (the devil) out of heaven. This is not to say that an angel could not have been involved; however, the Qur'an does portray it as being directly between God and Shaytan. It also implies that Shaytan accepted being thrown out insofar as Shaytan swore to misguide human beings until the end of time (and this oath implies that he is not trying to return to heaven).

Of course, God knows best!

A summary of what the angel Mika'il does as per Islamic texts can be found here: https://www.al-islam.org/ask/topics/4478/questions-about-Angel