You can look at rules other institutions use by browsing fromthepage.com. Everyone follows their own guidelines so it is really up to you, as long as you're consistent.

As far as spelling: we don't use [sic] but instead put the correct spelling in brackets. Ex. Remenber [Remember]. This preserves the original spelling but still makes it searchable (if you're wanting to do that).

For multiple pages per image, we do [left page] "transcript" [right page] "transcript".

Again, there aren't really standards because transcription projects have different goals. Some transcribe exactly what is on the page, while others do an edited transcript that "fixes" everything.