Checked hypothesis a) by idea i) looking up the standards. a) holds.
Details:
The current standard for email is rfc5322 (Internet Message Format).
Section 3.6 of rfc5322 states that from: and to: headers can only appear once.

The rules are given as 3.6.2.  Originator Fields
        from = "From:" mailbox-list CRLF
and 3.6.3.  Destination Address Fields
        to = "To:" address-list CRLF
in ABNF (rfc5234) which says: 
        Characters will be specified either by a decimal value
        (e.g., the value %d65 for uppercase A and %d97 for lowercase A) 
        or by a case-insensitive literal value enclosed in quotation marks 
        (e.g., "A" for either uppercase or lowercase A).
so the headers  fieldnames are case-insensitive.