Answers to assorted questions I've asked. Will update as and when. Thank you @Pentcheff and lab.
Many labels with the same species: yes, do transcribe them all (different from Herbarium practice), because each determination is someone's unique opinion so they're all valuable. link
Other duplicate information e.g. when one label is handwritten and the other is typed: just enter the unique information. link
Shore: this is part of the habitat. link
Accession numbers: two different ones, put one in the 'other' field (suggestion: the older, handwritten one) and one in the 'proper' field. In the more normal scenario of having two identical accession numbers, just transcribe the number once, into the proper field. link
Leg. before a person's name: this is short for Latin "legitis", "collected by"; i.e. the person after "Leg." is the collector. link
Things that would go in "Other" - some background info.
"When in doubt, use 'Other'." (link)
Bight '03 is (basically) an expedition: it's a big multi-location collection made by sanitation districts in SoCal around sewage outfalls. (link)
Collection method: yes, put it in Other. (link)
FID vs V; these stand for "Further Identification [is needed]" vs "Voucher specimen". Transcribe the circled/marked/indicated one.
ov.: probably an abbreviation for ovigerous, i.e. has eggs. Transcribe as written. (link)
yg: this is an abbreviation for "young". Transcribe as written.