Family Visitor Visa or General Visitor Visa

[uksettled]

I am settled in the UK with and Indefinite leave to Remain. I am inviting my father-in-law to visit me and family in the UK for less than 6 months. So the obvious visa type I need to apply is for Family visitor visa.
However do I need to submit copy of my wife's passport bio data pages to prove that he is her father. ie my father- in- law ? In my wife's passport, father's surname is mis-spelt slightly, it sounds almost similar. however different spellings. So if I need to prove that he is her father then we need to get a notary to certify that it is the same person. Is that acceptable ?

So in order to avoid the complication I thought I will apply in the General visitor visa category to invite my father in law. Is that a right thing to do? I started filling the online application and there was a question as ' Is the main purpose of your trip to visit/stay with a close family relative who is settled in the UK' ... I answered Yes. I clicked 'Create application'. Then it gave me a warning box saying- 'You have answered 'yes' to one or more of the visa selection questions. This means the visa type you have selected may not be the correct choice for your trip' . I clicked OK. and I clicked the 'Create application' once again. This second time it didn't give a warning and it let me proceed to the next step to fill the form.

So I am confused now whether I still can go ahead and apply in the General visitor visa category ??? Or is it compulsory to apply under Family visitor visa category as I am settled in the UK and he is my close relative???

I have all the proof ready and clear if it is just I am inviting my father- in-law in General visitor visa category.
So my questions are
1. Is it mandatory to apply as family visitor in this case ?
2. If yes ie, if I need to apply as family visitor, do I need to submit my wife's passport copy to prove the relationship ?
3. If yes, ie if I need to keep the copy of wife's passport, is a notary letter enough to say that it is the same person even though the surname is spelled differently?
Anyone gone through similar case and applied as general visitor and visa granted ?
Appreciate all your help !