Questions:
1) Do I have to get "approval of leave" letter from my employer to visit canada? I already got employment verfication letter,
--- EVL is enough.
2) Do I have to send all the previous H1B form 797, I20 I ever had ? or just the current H1B 797?
--- You have to send the Original current H1B, Passport along with original I-94. You have to send copies of previous H1B , I-20's, EAD card. Any document which proves the status you have/had in United States. (The consulate will return all the original documents along with the copies).
3) Question 8 on TRV application form asks about
Immigration status of applicant in country where applying ?
What should I answer here ? never had any status in canda ?
--- Immigrant status of applicant in country where applying - the country means from where you are applying for Canada Visa. As you are applying from United States, mention the current status you have in United States.
4) Which one is good option single entry or multiple entry ? (for visa purpose only I know money difference is there)
---If it is Visa purpose only then Single entry is preferred.
Note: A single entry visa allows you to enter Canada only once. This visa may also be used for repeated entries into Canada from the USA provided you do not enter another country.
Is there any checklist you guys can share for visitor visa ?
---Check this site for different Canadian consulates. Then check for the required documents in the consulate you wish to apply.
http://www.dfait-maeci.gc.ca/can-am/seattle/offices/default-en.asp
Basic documents are:
-Fully completed and signed Application for a Temporary Resident Visa (IMM 5257).
-Your valid, original passport(s), together with any previous passport(s) if it contains your latest U.S. visa.
-Proof of funds available for your stay in Canada (e.g. recent bank statements)
-Two (2) photographs, 35mm x 45mm (1.37" x 1.77"), for each family member requiring a visa. Please refer to our PHOTO SPECIFICATIONS for more information
-The correct processing fee, payable in cash (if paying in person, 8:00 to 10:00 a.m. ONLY) or by money order or bank draft. (check fee schedule, it differs for different consulate).Personal cheques and credit cards are not accepted.
-Valid, original I-94 card(s).
-If you have an H-1 visa for the U.S., you must submit the original H-1 notice of action (form I-797), a letter of reference from your current employer and recent wage statements/pay stubs. (copis of paystubs would be good)
-If sending by mail, Prepaid envelope with correct postage affixed. ( preferrable Express mail from USPS, next day delivery. It is usally $14.50. NO metered stamps)