Probably you can have your document limit listed in a lookup table to begin with, lets say Limit = 100 to begin with.
Then within your business process, add an if-else section, where the test condition is to evaluate current time against the XXXX and YYYY times you have in mind. If the current time is within your XXXX and YYYY, plus the limit is > 0 from the lookup table, you send the document and decrease the lookup table value by 1.
On the else condition, you just send the document without checking the limit value from the table, but update the lookup table again to your limit (100).
The key here is accessing and updating the lookup table value from within your business process.
Hope this makes sense.
You can create a custom schema based on the current schema you have listed in your business service that reads the file.
Once you have the custom schema, if its a specific ADT message you need the NTE segment on, then can create that message type from the base schema and edit the Doc Type Structure by putting ~[NTE] at the end of the raw structure.
Things might be on the same network, but on virtual machines spread across different data centers / physical servers, so definitely some level of HA for sure if you have more than one instance. You got to take and work with what's available. If in future, that client has network separation also available then you can increase your level of HA, but not having HA isn't right.