Does our conventional body belong to the external sense bases?

The adjective "interior" is often used for elements that are readily re-experienced. For example:

MN140:13.2: There are these six elements: the elements of earth, water, fire, air, space, and consciousness. And what is the earth element? The earth element may be interior or exterior. And what is the interior earth element? Anything hard, solid, and organic that’s internal, pertaining to an individual. This includes head hair, body hair, nails, teeth, skin, flesh, sinews, bones, bone marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, undigested food, feces, or anything else hard, solid, and organic that’s internal, pertaining to an individual. This is called the interior earth element.

So the sight of this page when your eyes are opened is external. But when you turn away and close your eyes to "see the page in your memory", that is internal. Internal perceptions are repeatable on demand.

An external earth element would be a mountain, for example. Skin and bones and mountains are just earth element, but skin and bones can be readily perceived in the mountains or at the seashore.

I have two parts in my question;

1. If we look at our own hand, leg, or any other conventional body part or if we touch and feel our own conventional body, do all these belong to the external sense bases?

2. If the answer is "yes" to the first part of the question, which form elements (rūpa) are included in internal?

For 1., whether its your own hands or other people's hands, from the frame of reference of your own eye-base's view-perception, those hands are all external bases. Similarly for your body-base's touch-perception when touching your own hand or others' hands.

For 2., the internal-bases/ajjhattikaAyatana are sensitive matter/physical support for the occurrence of the corresponded consciousness. In the example above, for the eye and the body bases, those would be the various physical sense receptors, nerve cells,... located in the eyes and the body. For further details, refer to CMOA Chpt. VII-35

OP: If we look at our own hand, leg, or any other conventional body part or if we touch and feel our own conventional body, do all these belong to the external sense bases?

Yes, if we see, hear, smell, taste or feel parts of the body, then those are experienced, hence they are a part of the external sense bases. They are part of "form".

OP: If the answer is "yes" to the first part of the question, which elements (rūpa) are included in internal?

No form elements are part of the internal sense bases, because the internal sense bases are part of "name", not "form" - they are related to feeling and contact.

The internal sense bases are the mental interfaces to the physical senses, from my understanding.

‘The six interior sense fields should be understood.’ That’s what I said, but why did I say it? There are the sense fields of the eye, ear, nose, tongue, body, and mind. ‘The six interior sense fields should be understood.’ That’s what I said, and this is why I said it.

‘The six exterior sense fields should be understood.’ That’s what I said, but why did I say it? There are the sense fields of sights, sounds, smells, tastes, touches, and thoughts. ‘The six exterior sense fields should be understood.’ That’s what I said, and this is why I said it.
MN 137

And what is feeling? There are these six classes of feeling. Feeling born of contact through the eye, ear, nose, tongue, body, and mind. This is called feeling.

And what is contact? There are these six classes of contact. Contact through the eye, ear, nose, tongue, body, and mind. This is called contact.

And what are the six sense fields? The sense fields of the eye, ear, nose, tongue, body, and mind. These are called the six sense fields.

And what are name and form? Feeling, perception, intention, contact, and attention. This is called name. The four primary elements, and form derived from the four primary elements. This is called form. Such is name and such is form. These are called name and form.
SN 12.2

Name and form is mind and body.

The idea that the six interior sense bases refer to the mental interfaces to the physical senses is supported by:

The six interior sense fields, reverends, are one end. The six exterior sense fields are the second end. Consciousness is the middle. And craving is the seamstress, for craving weaves one to being reborn in one state of existence or another.

Name, reverends, is one end. Form is the second end. Consciousness is the middle. And craving is the seamstress, for craving weaves one to being reborn in one state of existence or another.
AN 6.61