For the sake of simplicity I interpret: regards material form as self
is basically I am a collection of atoms
The wrong view is: "The body is a/the self; I am the body; the body is me".
The right view is: "The body is a collection of atoms (elements/dhatu)."
self as possessed of material form: this is hard for me to understand,
maybe as eager to have many houses and fat bank account?
Yes, the above appears correct. The wrong view is: "I have a body; I own gold & silver".
self as in material form: hard to understand.
The word "form" ("rūpasmiṃ") in Pali, above, is in locative case. The above appears to mean to regard the self as located in the body. Possibly, for example, "my liver; my finger; or my heart"; or the common belief: "the soul/self abides in the body".
In Hinduism and also probably later Buddhism, there is found the terms "dehi" and "deha". "Deha" means the "envelope or shell" the "dehi" dwells in. "Dehi" means the "soul" or "creature" dwelling in the "deha". For example, in the Bhagavad Gita:
vāsānsi jīrṇāni yathā vihāya navāni gṛihṇāti naro ’parāṇi tathā
śharīrāṇi vihāya jīrṇānya nyāni sanyāti navāni dehī
As a person sheds worn-out garments and wears new ones, likewise, at
the time of death, the soul casts off its worn-out body and enters a
Or the Christian Bible says: "You are the body of Christ. Each one of you is a part of it."
material form as in self: I can be only certain about things that are
taking place in my head?
I assume the word "self" ("attani") above is also in locative case. This appears to mean believing: "I am made of mind & body and the body is a part of myself".
For example, when my uncle passed away, at the funeral, i ended up in the car of the Christian Orthodox Priest travelling to the cemetery. The priest said to me it was wrong that my father (an Orthodox) was cremated because in the Orthodox religion they believe the person is made up of both the soul and the body rather than merely the soul. Therefore, it is improper for an Orthodox to cremate the body of the deceased. The view of the priest here was the self is made up of two parts and the body is part of the self. This appears to be an example of: "form in the self".
The Pali is below:
rūpaṃ (nominative or accusative) attato samanupassati, rūpavantaṃ vā attānaṃ, attani vā rūpaṃ (nominative), rūpasmiṃ (locative) vā attānaṃ.
They regard form as self, self as having form, form in self, or self in form.