18 year old is an adult now so regardless of whether the courts deemed you as accepting him as your own or not- he is an adult.
The two younger ones are still both in school and as your wife was working while they were not and only recently quit will probably go against her. Most judges today will say both parties need to contribute financially.
It really will be fine. Just stand your ground with regards to disclosing your partners finances as she isnt party to your divorce.
And put the offer to her. Dont accept anything you are not happy with. It sounds to me like you are being more than fair.
Finally, dont accept something just to see the end of it. You will only be bitter with yourself.
OVer 18s in work are considered independent. Courts don''t discriminate between natural children of the relationship and other children who lived with the divorcing couple. Children under 18 are a priority but children over 18 in education aren''t "irrelevant" and it''s reasonable for them to be provided with a base even if they don''t live there full time.
I''d also echo what has been said to you by Fiona and somuch. I would even go as far as to say that your partner''s finances will be pretty much ignored by a judge, which is exactly how it should be. My husband (then partner) lived with me at the time of his Ancillary Relief (2.5 years after separating). Even though I earned a good wage and he lived in my home, this did little for his ex other than gave her a mesher order. She had originally been offered this by my husband but had refused as she wanted 100%.
I did hand over my income details, but in limited form, as I was advised that if I didn''t a judge could ''imagine'' what I earned. However, I think I really should have stood firm and said no as this was none of his ex''s business. I have read many posts on here where a partner''s income has been demanded by a solicitor but none where it has actually been ordered by a judge.