1) Can I make Tax Invoice on ms excel and file GSTR manually ? (though you have touched the matter on your post)?
You are free to make invoice using MS Excel or any other software as long as you invoice fulfills invoicing requirements prescribed under GST rules.
You can file return manually on GST portal. No software is required. I will suggest doing it number of invoices is low. Otherwise it will be huge task. You can also program your excel in a way that monthly billing details can be automatically arranged in return filing format. You can do it yourself.
2) How could i transfer my closing stock (as on 30.06.17) to my GST account ?
I believe you are asking about input credit on stock lying as on 30 June 2017. Government issued transition rules. Once you file your June quarter/month return, stock disclosed in return/input as per return can be carried to GST.
3) I works with a mfg unit where Electrical switches are manufactured. 1 unit switch is made of 30 various products. Then my query is can I maintain stock besides accounts in tally ? If yes then how could I maintain break-up wise stock in tally ?
Tally can be used for invoicing, accounting and stock. I understand it freezes a lot and report generation takes time.
You can use tally only for billing or accounting or stock or for all purpose.
You can also try the invoicing application which I have coded myself. It is being used by many people to generate GST invoices and maintain stock details.