Does it work like this?

ClassMethod WriteCapture(vstrCommand As %String) As %String
{
    set tOldIORedirected = ##class(%Device).ReDirectIO()
    set tOldMnemonic = ##class(%Device).GetMnemonicRoutine()
    set tOldIO = $io
    try {
        set str=""

        //Redirect IO to the current routine - makes use of the labels defined below
        use $io::("^"_$ZNAME)

        //Enable redirection
        do ##class(%Device).ReDirectIO(1)

        XECUTE (vstrCommand)
        
    } catch ex {
        set str = ""
    }

    //Return to original redirection/mnemonic routine settings
    if (tOldMnemonic '= "") {
        use tOldIO::("^"_tOldMnemonic)
    } else {
        use tOldIO
    }
    do ##class(%Device).ReDirectIO(tOldIORedirected)

    quit str

    //Labels that allow for IO redirection
    //Read Character - we don't care about reading
rchr(c)      quit
    //Read a string - we don't care about reading
rstr(sz,to)  quit
    //Write a character - call the output label
wchr(s)      do output($char(s))  quit
    //Write a form feed - call the output label
wff()        do output($char(12))  quit
    //Write a newline - call the output label
wnl()        do output($char(13,10))  quit
    //Write a string - call the output label
wstr(s)      do output(s)  quit
    //Write a tab - call the output label
wtab(s)      do output($char(9))  quit
    //Output label - this is where you would handle what you actually want to do.
    //  in our case, we want to write to str
output(s)    set str=str_s   quit
}

set obj = {"RecordIDs":[{"ID":"1234","Type":"INTERNAL"},{"ID":"1234","Type":"EXTERNAL"}],"ContactIDs":null,"Items":[{"ItemNumber":"320","Value":null,"Lines":[{"LineNumber":0,"Value":"1","Sublines":null},{"LineNumber":1,"Value":"100063287","Sublines":null}]}]}
w obj.Items.%Get(0).Lines.%Get(0).LineNumber
>0
w obj.Items.%Get(0).Lines.%Get(1).LineNumber
>1

To iterate over arbitrary number of array elements use iterator:

set iterator =  obj.Items.%Get(0).Lines.%GetIterator()
while iterator.%GetNext(.key,.line) { w line.LineNumber,! }
>0
>1

You'll need to subclass EnsLib.File.InboundAdapter and modify this line in OnTask method to do what you need:

 $$$LOGINFO("Skipping previously processed file '"_tOneFilename_"'")

Alternatively you can monitor done file table and send alerts when you find new records.

Both options seem lacking.

There are several ways to handle this:

1. Dynamically adjust BP settings (your suggestion). To do that you need to modify object(s) of Ens.Config.Item class and update the production. Check GetItemSettingValue method in Ens.Director class for an example of getting setting value. After that update the production and that's it. The problem with that approach are:
   • Updated settings take effect for all new incoming/outgoing messages
   • Updating production is a time-consuming operation, it should be used as rarely as possible
2. Proxy process. Add proxy process that calls your target process. The proxy process should only contain logic related to error processing. And the target process should return the message immediately without any error checks.
3. Modify the process so it does not return an error each time, but rather sleeps and calls the operation again depending on an error.

Code sample for 1 - utility method to search for setting in a production object.

/// Find BH in a current production by a setting value
ClassMethod findConfigItemBySettingValue(settingName As %String, settingValue As %String, businessType As %String = "", enabledOnly As %Boolean = 0) As Ens.Config.Item
{
    #dim sc As %Status
    
    // Get current production object
    #dim prod As Ens.Config.Production = ..getCurrentProduction()
    if '$isObject(prod) quit ""

    // Cycle over production elements
    #dim item As Ens.Config.Item
    #dim result As Ens.Config.Item = ""
    for i = prod.Items.Count():-1:1
    {
        set item = prod.Items.GetAt(i)
        
        // Search only for specified type
        if ((businessType '= "") && (item.BusinessType() '= businessType)) || (enabledOnly && 'item.Enabled) continue
        
        // Cycle over settings
        do item.PopulateModifiedSettings()
        set ind = ""
        for
        {
            set setting = item.ModifiedSettings.GetNext(.ind)
            if (ind = "") quit
            
            // Found it?
            if (setting.Name = settingName)
            {
                if ($ZStrip(setting.Value, "<>W") = $ZStrip(settingValue, "<>W")) set result = item
                quit
            }
        }
        
        if $isObject(result) quit
    }
    
    quit result
}

/// Get current running production object
ClassMethod getCurrentProduction() As Ens.Config.Production
{
    #dim sc As %Status
    #dim prodName As %String
    #dim prodState As %Integer
    
    // Find the name and status of current production
    set sc = ##class(Ens.Director).GetProductionStatus(.prodName, .prodState)
    if $$$ISERR(sc)
    {
        $$$LOGERROR($System.Status.GetErrorText(sc))
        quit ""
    }
    
    //Status should be "Running"
    if (prodState '= $$$eProductionStateRunning) quit ""
    
    // Open object by name
    #dim prod As Ens.Config.Production = ##class(Ens.Config.Production).%OpenId(prodName, , .sc)
    if $$$ISERR(sc)
    {
        $$$LOGERROR($System.Status.GetErrorText(sc))
        quit ""
    }
    
    quit prod
}