I don't have the Mac; but I'll try to answer your questions
1) The half-sibling question. What you've described is exactly what you should have done. They are now half-siblings in your database. Check the Relationship calculator, wherever it is, for the Mac - in Windows it is on the Main Menu under Tools. They should show there as half-siblings.
(If you want them to both show in the second spouse's family in the Family Tab of the People Space, then you have to make the first child be a stepchild of the second parent - which is another matter. In Windows, you would add the parent/child to the child/parent at Persons > Attach/Detach and then go to the Person page, click on the Relatioship button in upper right and designate that child/parent relationship as step.)
2) As for the adoption:
a) I assume you already have Helen Smith entered as daughter of Joe and Ann. And you have entered Fred Miller as second husband of Ann.
b) Now, add Fred as father of Helen, or Helen as daughter of Fred.
c) Go to Helen's Person tab, click on Relationship view - buttons in upper right.
d) She will show 2 biological parents - change father Fred to adopted father status.
e) Click on the "FACTS" view - button in upper right.
e) Click on the "plus" button and select to add a NAME fact. Enter Helen Miller and ok
f) Right click on the name and check it as preferred.
g) Click on the "plus" button to add an adoption fact. Enter place of jurisdiction of the court as place and the date and case number in description.
h) Add a citation. Right hand panel. You can look at the templates to see if there is one setup for legal cases / adoptions - otherwise, just put in what seems logical to you.
h) Scan adoption papers. Add the adoption papers to the citation media (automatically also becomes Fact media, as well.)
If you want the adoption fact to be hidden from printing in reports, right click on it and make it private. Then check in reports to not print Private Facts. You can do that for one fact or globally at Manage Facts. For example, you can make all those Social Security Numbers downloaded from the SSDI database private by making the Social Security Fact private. Although I think you may have to go back after new additions to ad the new additions to be private.)
Whew... that took more steps to write out than I thought. It seems complicated when reading/writing, but once you do it, it seems intuitive.