Having worked for a peer-reviewed journal and as editor of special issues:
'Forthcoming' shouldn't actually be used until it has passed peer-review and minor revisions, etc., been finally accepted. So anybody who is using it to describe work simply submitted is wrong to do so.
'In press' means it is actually in press! i.e. you have returned corrected proofs. So in essence, this can only be used if the article is in the next issue of the journal.
'Accepted for publication in ...', I think is fine for stuff that has passed peer-review.
Another option I have used is 'Scheduled for publication in ...' - this works best if you know the volume, issue date and the estimated date of publication. Although be prepared for the publication schedule to change sometimes due to reasons beyond the editor's control.
As soon as you have vol. no, issue no., page numbers, publication date (usually just the month) and a DOI (digital object identifier) you don't have to qualify it at all. It fully counts as a publication. But this tends to only be the case when publication is imminent.
Hope this makes some sense.