Mahārāhulovāda Sutta MN 62 explains there are internal and external form. Your understanding is correct about internal and external forms.
Literally the body consists of name and form where the form is made of four great elements. Generally parts that belong to the earth element are solids, similarly, water: liquid, air: gaseous, fire: heat. This is likely to be your idea about the explainations in EBTs discourses.
Abhidhamma texts explains about suddattaka (octats) which consists of above four elements and another four qualities color (vanna), odor (gandha), taste (rasa), and energy/sap (ojā). When it comes to octets no matter the form is internal or external it consists of octets (pure-octets??) . There is no contradiction but an explaination which cannot be found in basic texts to my knowladge. However, if you take examples used in the sutta which can be understood as patavi, āpo, so and so, this later explaination is somewhat contradictory. In my opinion the blessed one took these parts as examples to point the dominant characteristic element of each body part. If that is the case, considering hair under earth element does not necessarily mean it only consists of earth element. But, who ever starts his meditation with a hair or bone may mostly experience arising and ceasing of earth element.
As explained in satipaṭṭhānasutta one can start his meditation from some visible touchable and rough (olarika) object (ex: breath, hair, a dead body), but with the practice his object become subtle and gentle.
As stated in MN 62:
Whatever is an internal liquid element and whatever is an external liquid element, just these are the liquid element.
By means of perfect intuitive wisdom it should be seen of this as it really is, thus: This is not mine, this am I not, this is not my self. Having seen this thus as it really is by means of perfect intuitive wisdom, he disregards the liquid element, he cleanses his thought of the liquid element (Same to the other elements).
evametaṃ yathābhūtaṃ sammappaññāya daṭṭhabbaṃ
By means of perfect intuitive wisdom it should be seen
This particular aspect is about the basic level of the form. This can only be seen through perfect intuitive wisdom. These levels are not visible or sensible to our physical senses. No matter which object you select from satipaṭṭhānasutta, when you develop your samādhi to achieve perfect intuitive wisdom, you can see arising and ceasing (uppādavaya) of name and form (nāmarūpa), where you experience arising and ceasing of one of these basic elements. (Just ignore if you don’t understand, I am not gonna explain this further.)
To explain the subtle and basic nature of elements commentators used octets and many other explainations for mental part of a being. But, these are not necessary to understand subtle nature of each element.