Contracts is the established terminology, but it's confusing the first time you encounter it.  To the non-Bitcoin-initiate, a contract is a legally-enforceable agreement that might involve lawyers and other complications.  That's not a good description of basic Bitcoin contracts, so I choose to call them collaborations in this opening paragraph.

With all the shuffling around, we now use the word contracts seven times in the transaction section before first defining it in the contracts section.  I think this is sub-optimal.  I think we have three options:

1. Define what a contract is in this sentence or, at least, in the following paragraph.

2. Add an extra word to indicate we don't mean standard legal contracts.  For example, "[...] and complex **Bitcoin** contracts."

3. Remove all occurrences of contract in the transaction section.

I like option 3 best, followed by option 2.  If we don't mention contracts until the Contract section, readers won't be confused by its non-typical definition.  The main use of contracts in the transaction section is in the malleability subsection; that subsection should probably be split anyway so the parts of malleability which apply to atomic transactions is separated from the parts that only apply to non-atomic transactions.
 
If you agree, let me know if you want me to do the split/re-write on the malleability subsection.