I didn't see your original post or I would have replied.
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We are in the middle of our immigration journey and just had our I-130 approved.
I will start with your original posts, just so others have the info. For the marriage certificate, the just needed a copy of the true certificate. Some places will give you a memorative copy but this isn't the real thing. Only a copy is needed, nothing needs to be certified.
For your passport, you are overthinking it and there is no requirement to show your other passport if you dont have. A copy of your ILR alone is fine.
You shouldn't do a cover letter. They want the specified documents only. If you provide something extra, they will pull those documents and bin or return to you. Particularly when they are backlogged, as they have been until the last 2 weeks, they were definitely returning extraneous docs with a big red naughty stamp.
The money is the most important thing. You will get an email (if you signed up for email alerts) but they come 2-3 weeks after submission.