FLR - so many questions

Hi everyone - hopefully I can get some advice from the collective fountain of knowledge that is this forum. 

I'm an American in the UK and my [spouse] VISA expires on 8 August, 2019.  I submitted/paid for my application yesterday so that bit is done, so now my husband and I are scrambling to find the right documentation, etc.  I work for my local council and have been getting various emails from the recruitment department regarding my visa and needing to show them "proof of postage" prior to 8 August, or else my contract may be void. 

When applying yesterday, I got a bit confused about this proof of postage however, so just wanted to clarify a few things. 

So I've submitted my application on 14 July, 2019.  My husband and took the first available appointment to the UKVCAS centre in Croydon for Monday, 5 August, 2019.  I have to scan in my supportive evidence and when I go there, they will take my finger prints and my picture. 

And.....then what?  Will I get further directions at that appointment of where to send my paperwork off to (preferably the next day so that I can provide the "proof of postage" to my employer?)  Is there anything I can get prepared ahead of time so that we could potentially mail my stuff off immediately after the appointment? 

Also, can documents serve as....two things?  For example, if we have some mortgage papers we'd like to provide for housing costs and things, but they were mailed to us and would be good show as evidence that we live together, could that piece of evidence be used doubly? 

Also, a question about bank statements.... we are with NatWest so if anyone else is, you know the tiny sheets of paper they send with your statement information on them.  Does it matter that our address isn't listed individually on the sheets of paper?  At the beginning of each statement in the envelope, there is a piece that shows it's been sent to us - can these be proof as they don't specify a date on them?  I'm assuming no. 

Okay last question (I think) in regards to my husband's pay slips.  He works for Royal Mail and gets a payslip every week.  When looking through the last year (we are starting from 1 July, 2018 to most recent) he is missing 2 payslips.  He contacted the HR offices and they sent him an official letter on their stationery with this huge spreadsheet of a of his payment information dating back to 2017.  We are going to write a cover letter explaining this along with the letter/spreadsheet that will show the missing payments.  Do we think this is okay?  They are being very difficult and won't give us copies of the exact payslip that was originally sent to him. 

Whew, okay.  That is all for now. 

Firstly, never submit your application until you have all documentation to hand.

You don't send off any documentation, hence them scanning it all. You keep all your documents. You should have received a document showing proof of application payment and application complete.

Using a mortgage statement as two documents is fine but there is no window to say 'please use mortgage statement as proof of house ownership and correspondence' in the application if i recall. If there was, it's too late now as you've already applied (just reiterating an example of why you should never apply until all paperwork/documents are done). You could scan and upload a cover letter asking them to use it for both?

Regarding Natwest i'm having trouble imagining what you mean. Can you upload a picture obviously ensuring you cover all important information?

I can't answer the payslip question unfortunately.

Regarding your employer, ksand sent me this a few weeks back regarding that

As long as her application is made before her current visa expires, she will be given Section 3C leave, meaning her current immigration status and right to work will continue (same as when she applied for FLR(M)).

See: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/771652/3C-3D-Leave-v9.0ext.pdf

From that file:

Section 3C leave extended when an in-time application is made

An in-time application is an application made by a person in the UK who at the time of application has leave to enter or remain.

Where an in time application to extend or vary leave is made and the application is not decided before the person’s existing leave expires, section 3C extends the person’s existing leave until the application is decided (or withdrawn).

Section 3C does not extend leave where the application is made after the applicant’s current leave has expired.

If her employer is concerned about her right to work in the UK, they can use the Employer Checking Service here:
https://www.gov.uk/employee-immigration-employment-status