Work contact executed prior GST but partly work done after GST implementation

Good day sir,
I have entered into agreement with a Architect for construction of my Duplex independent house prior implementation of GST. Labour as well as Material is to be arranged by him. Nothing is mentioned about applicable tax in agreement, However it is written in agreement that house possession will be given in " ready to move condition" and 1500₹/ sq ft will be charged. The total contact cost was about 40 lac.

He carried out half work pre GST and remaining work post GST. But he haven't paid any GST as till now he is not having GSTIN, Additionally he have bought most of the material post GST without paying GST by taking fake bills to maximize his savings.

As he have not taken proper GST invoices, now I cannot claim guarantee on any of my house fittings.

I want to know 3 things :
1) if fixed rate is agreed per sq ft - it is implied that applicable taxes are included - right.
2) if I file a complaint against him in GST for providing fake invoices , will any action come against me.( means do I have to pay GST) it should be his responsibility as he bought material.
3) will any action come on me for not paying GST on contact amount although I have already paid him flat rate which includes taxes ( though not expressed in agreement) and he is also not having GSTIN.

Early reply will be highly appreciated.