OCI application for minor and spouse

Hi, I'm an existing OCI holder and I need to apply for OCI for my wife (UK born and British citizen) and twin babies (both UK born and British citizens). I'm based in London and planning to apply to VFS Goswell.
My questions are a bit detailed so I thank in advance profusely to this forum for the invaluable help. Of course I'd be more than happy to return the favour in future. My questions are:

A. OCI for Minor
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1. Does the UK issued original birth certificate require apostilation?
2. Does it additionally require attestation by London High Commission?
3. How do I prove residential address for my babies? Will a statement issued by a UK investment broker be OK? Or a GP letter?
4. Are minors expected to be present during application submission at VFS center?
5. I have struggled to take their fingerprints - somehow I managed to upload a version - but how strictly do they check for clarity of prints?


B. OCI for Spouse
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1. Does the UK issued original marriage certificate require apostilation?
2. Does it additionally require attestation by London High Commission?
3. Where is the Spouse interview conducted? Is it at VFS center or in Indian High Commission? I have read both versions online so a bit confused.
4. Is the spouse interview done during the VFS appointment time itself or is there a separate appointment needed?


C. Documents on the basis of which OCI was issued to me
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1. Are the following sufficient?
(a) OCI booklet scan of first and last page
(b) OCI lifelong visa sticker scan
(c) Indian passport surrender certificate
(d) Indian passport
2. I got my OCI in 2013, at that time I did not receive anything other than surrender certificate. Is there any other document required as well? I read somewhere about renunciation certificate? I'm positive I did not receive that.
3. Do I need to submit my Indian birth certificate as well? I do not believe I have that.


D. Misc
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1. Is it really true that apart from UK passport, all other originals have to be left with VFS? I.e. marriage certificate, birth certificate, proof of address (driving license) etc?
2. I have booked a family appointment for all 3 applicants - my wife (spouse OCI) and my twin babies - to return the original documents - should I leave 3 self addressed envelopes or one large one would be enough.

D3. I recently renewed my british passport, so the previous passport that was linked with OCI is now no longer valid. How critical is it to update the link to the new passport? As and when I will arrive in India are the immigration officers likely to expect me to provide the originally linked British passport along with OCI booklet? How strictly is this checked?