Hi,
Answers are as below:
1) Since you said that ITC is already availed (though wrongly) under ISD, now the same cannot be availed twice. As such you can avail benefits of regular registration including ITC only post Feb-18. Means ITC available after regular registration and taxable supplies after regular registration shall only be eligible. No ITC for previous invoices shall be allowed now.
2) If any invoices are for your HO which is MH and are dated before your regular registration, the same can be availed now. But the same has to be transferred on current date only from HO MH to MP and to be shown as consumption after that only. This I believe would not be possible.
3) The best remedy would be to utilize the ITC (if they are prior to regular registration date and in the name of your HO) to transfer the same in MP on current date and consume the same in MP from current date onward.
Regards,