Yes service tax paid in June can be used as itc in gst for this you have to file TRAN1 and must have credit c/f in June service tax return
1 calculation of gst is 100/118*18 coz 100 includes 18%
2 yes credit is allowed for all inputs/services / capital goods which are used in the course or furtherance of business subject to negaolist of itc
3 calculation of gst liability is month on month basis , means July output tax can be adjusted with July input tax
If net balance is payable then you have to pay and if net balance is excess then you can carry forward the same
4 your answer in point 3
5 in gstr1 if you rendered service to Registered person then then fill in table 4
If unregistered in other state and value is more then 2.50 lakhs in single invoice the. Fill table 5, unregistered in same state or in other state value less then 2.5 lakhs then use table 7
Hope ur queries now resolved