My opinion on your queries are as under:
1) Can a perjury case filed against wife who had signed false affidavit in the court that she is unmarried ?
Opinion: It may be filed.
#2) Also will she get imprisonment under sec 193 or just a fine amount ?
Opinion: Its depend on the court, the court may punished by imprisonment or by fine only.
#3) Is this a perfect ground for marriage annulment for second husband ?
Opinion: No, this is not a perfect ground to get annulment.
#4) Can a cheating case(IPC 415, punishable under sec 420) filed against second wife & her close relatives ( e.g. brother,sister,mother,father,uncle) as all of them were involved & hidden the information ?
Opinion: No, it can not.
#5) What are the chances of imprisonment under sec 420 to all those involved if applicable or would it be fine amount only?
Read section 420 of IPC
420. Cheating and dishonestly inducing delivery of property.—Whoever cheats and thereby dishonestly induces the person deceived to deliver any property to any person, or to make, alter or destroy the whole or any part of a valuable security, or anything which is signed or sealed, and which is capable of being converted into a valuable security, shall be punished with imprisonment of either description for a term which may extend to seven years, and shall also be liable to fine.