Well I'll basically look at this in two ways first of all evil spirits are real and can cohabitate anybody I believe that's the reason why some christians have the mentality that they need to be delivered from it there's a bible verse that says "suffereth not a witch to live" you must know that the human body can be unclean as a result of possession by devilish spirit just like we saw in numerous places in the bible where Jesus had to heal people that had unclean spirit is an evidence to show that unclean spirits can dwell in people however what I definitely do not concur to is having to go through improper method of cleansing in order to get rid of evil spirits, some doctrines even impose flogging of people with canes , burning of candles as well as other nefarious methods in order to cast out evil spirit.
Total deliverance from evil spirit can only be achieved or attained through prayer and fasting and not through deliverance by some who call themselves men of God prayer is the only way through which god can great people and that's what the bible says that christians should the way of false prophets and false teaching in the name of god because this is a sign of the end of days and had people been reading their bible well they would have spiritual knowledge of what the bible actually says rather than what their spiritual leaders says so I think this is as a result of ignorance of the word of God that is why people subject themselves to this unholy deliverance processes which is not of God.
So like you pointed out the bible says light and darkness cannot exist in the same place so it is difficult for an evil spirit to cohabitate with the spirit of god in essence the more reason why we must study the scripture understand its meaning and its purposes so as not to be misled by the prophets of this current days the bible warns christians watch out of the end of days and it signs because even darkness can appear as light and even the devil can be transformed to light so that's why christians must watch out and no evil cannot coexist with light.
That's a very good question.
I never heard this one before.
If I were to put my Christian Hat on for a brief moment, I would say this:
To receive the blessing of the holy spirit is to actively pursue one, a believer may feel that they fell victim to temptation and blocked the entrance for the holy spirit of god, simultaneously allowing for evil spirits to take a possession of them. Hence the ritual.
But you're right, like many things of religious nature, it wouldn't make sense.
Religion is, inevitably, open to interpretation.