(a) I have one card with good credit history. I have been told one should have 2-3 credit lines. If I apply for another card now how long should I wait to apply for a mortgage loan.
A new trade line will not be rated until after 6 months of account activity have passed. A new account, however, will reduce your credit score slightly. The "2-3 credit lines" is a rule of thumb. Some lenders may require; others will not. Fannie Mae, Freddie Mac, and FHA do not have minimum tradeline requirements.
(b) My spouse has no credit card, so no credit history, never had it. She would be a co-borrower. I have been advised that we can go ahead to apply for a loan together, without her having a credit card. We need to account for both of our salaries though.
FHA insured mortgages do not require a credit history reported to the a credit bureau. Many lenders, however, won't write FHA insured loans based on non-traditional credit (eg rent, utility payments, etc)
(c) My spouse should get a credit card from my credit card co. as an 'authorised user' through me, so that my credit card history gets shifted to hers.
Authorized User tradelines no longer count in developing a credit score, thanks to companies which aided fraud by "renting" tradelines from people with good credit to people with poor credit.
(d) My spouse should apply for an independent credit card through me from my credit card co. so that my credit card history gets added to hers..
To count towards your wife's credit score, the account must be either "joint" (both of you apply) or individual (she applied on her own). Bear in mind, though, that any new tradeline won't count towards score until 6 months of activity have passed.
(e) My spouse should get an american express/costco card on her own.
It won't matter if the account is with AMEX, Cost Co, or a bank. All are considered revolving accounts in developing a credit score.
(f) my spouse should apply for a credit card from her own bank or take a secured credit card.
What is our best option so that we are able to obtain a loan at least rates,
There is essentially no difference between a bank issued credit card or a secured credit card. Many banks offer secured cards. BE SURE, though, that any secured card for which you apply reports account activity to all three credit bureaus. Many do not. And many will charge you numerous junk fees at application.
My advice to you is to avoid doing anything with credit until after speaking with an FHA lender.