Income tax

One year before the deal and two years after the deal a new house can be purchased.

You can claim LTCG tax exemption.

So with only the POA document from the buyer can i invest the money or should i wait till the buuyer registers the property on his name.

There is a practical problem in this case. The practical issue is circle rates. They may differ from the time of original registery date and date of POA. This difference create problem. If you calculate capital gain now. Then what will happen at the time of registery you will have to pay full tax on difference amount. Secondly previously people were doing this by showing cash receipts of the amount in diffrrence which is now prohibited under new laws. And how the difference between the two will be meet out And what about TDS provisions if applicable

once capital gain arises by transfer of property you can invest the gain for 54 deduction. here you have alredy been transfered right of property, means a legal possession is given, which is the part performance of contract ,considered as transfer. so you can invest the gain.

Expenditure of administrative nature incurred during construction period should be apportioned to fixed assets on commencement of plant or should be charged to revenue or should be deferred as pre operative expenditure to claim in next 3-5 years ?

The money got from sale of property should be put in Capital gains saving account.[CGSA] 

The new House should be purchased from this account only say in three four installments.

You have proof that capital gains was put in CGSA . You have proof that new House was purchased from money in CGSA.  That is sufficient for IT dept that CG was invested in new House..