I don't have a specific answer to your questions, but here are my thoughts:
1. That looks exactly like German in a sense:
vier=und=zwanzig
four and twenty
'twenty four'
2. There are languages with a true 'and' conjunction and others that use 'with' for that function. See:
Stassen, L. (2000). AND-languages and WITH-languages. Linguistic Typology, 4, 1–54.
http://dx.doi.org/10.1515/lity.2000.4.1.1I wouldn't be surprised if in this language 'with' is used more generally as a noun phrase conjunction.
Then again, I don't know whether this is otherwise common in numerals. It might be.
3. If this is a kind of coordination, then I wouldn't be too surprised by the ordering.
4. This isn't the same as syntactic modifier-head ordering. In German at least it's a kind of compound. While as a general expectation you might think the order would be the same, an exception here doesn't stand out as surprising to me. It does look like, I think, the "head"/base is "ten" and modifier is "five".
5. I know of no special term. I'd refer to it as the "units" place as opposed to "tens". But that's not a linguistic term.
6. In general, what is the order in compounds? Cross-linguistically, I believe there is a tendency for modifier-head order, actually, but you may want to check on that. So in that case, I'm not sure why this would be an exception in that sense, though it isn't the same as the syntactic system.
