Hello Keyur,
- Your logic seems fine. But I think you should use Requested Date field at Item level of a sales order for comparison. As this is the Delivery Date requested by customer. The dates under schedule lines are after Availability check.
- Now the question is, Where will you compare those two fields..?
You could direcly made a delayed SO report if any indicator was there on delay. But I think it is not there.
For comparison, You are only left with 1 option, i.e extending the SalesOrderItem datasource with the "delay" indicator using your Bydesign studio. If you are comfortable with that then you can easily compare the two fields inside the extended SO business object. You can make that indicator field true if requested date is less than actual delivery date. Based on that datasource you can extend an existing SO report, where this datasource is used.
- The other , but unoptimized way would be going with complete custom report. which is not advicable.
If you have any other idea, then please let me know. 
Regards