Navaraatram (नवरात्रम् - नवानां रात्रीणां समाहारः)

Navaraatram (नवरात्रम् - नवानां रात्रीणां समाहारः)

I post a question here about Navaratram - we do it at 3 levels

1. Custom level (Sampradaya) - there are multiple customs - some do pooja in group, some do it in the road, some strictly inside the house, some dance-some don't, some eat sweets, some don't, some don't eat rice, some don't lock the house for all the 10 days, some travel, some observe total fast for all the 10 days with just water.

Do we know why these?? and what is the purport behind these customs?

2. Cultural level (samskriti - there are 3 prayojanas) - here pooja is with Kalasha, or with Mukha, or with Yantra, etc. Poorna vratam is observed, partial vratam such as abstaining from some foods are observed, some kind of Japa is done or sahasranama is recited. what are all the significance of all these ? - what is behind this culture?

3. Philosophical level (taatparyam) - what the purport behind the metaphorical story and the alibis?, what is the highest truth behind the Navaratram. Is there a connection between Gurupoornima and Navaraatram?.

Do people know this festival is actually celebrated at these 3 levels

Who is Mahisha ?,
Why there is no 'Devi' on the 10 day?
Why worship Saraswati on the 9th day? and not in the beginning ?,
Who is Raktabeeja? why Kali /Durga to kill him? - who is Durga here?
Why it is not called as Dasha raatram?
(In North it is wrongly called Dasharatra then it is corrupted version nowadays as 'Dashara',)
Why it is ratri and not day? - what is the significance of night with respect to Mahisha ? (Pl. don't say demons are strong in night - that is a lame answer - the Tatva behind - what is it??)
Why pulses are only (you're supposed to) eaten on all the 10 days?. What is the deep significance the whole festival?

Please note the customs change greatly for every state in India, the culture remains the same at deep level and the philosophy is only one.