In the Insert Ciration window, click on the blue bubble for the citation and choose Suppress Author. Then manually type the author names in the text of your document.

(Note that APA specifies you should use "and" rather than "&" when typing authors in text like this.)

not really, no. But the way you'd delete the citation would just be to delete it in the Word document anyway, so I don't really see how that would be any different depending on whether the authors are part of a Zotero-generated citation or written out by you.

(We may eventually add an option for these types of citations and there are some reasons -- such as adjust et al -- you may want them to be generated by Zotero, but that's not going to be any time very soon).

CSL (the programming language that Zotero uses for formatting citations) doesn't support this type of formatting currently. As @adamsmith mentioned, there is a possibility that this support may be added in the future, but that will not happen very soon. There are a lot of details about various formatting options that need to be considered before support can be added. For comparison, in my experience, non-CSL citation software that does support this type of formatting (e.g., Endnote), doesn't do so particularly reliably.

That code will produce this in the case of multi-source citations:

Johnson et al. (2006); Smith & Jones (2005); Harper (2006)

which is really not helpful in any situation. You can certainly create code like this, but it will be very inflexible (e.g., as you can see, it would be hard to handle multi-source citations well). Also, as you note, you cannot have both Author (Year) and (Author, Year) style citations automatically formatted in the same document. I think that cases where someone would want only Author (Year) style references are extremely rare, so Zotero and CSL's current default behavior for styles to support (Author, Year) automatically and to do Author (Year) through suppress-author is probably the most efficient.

Certainly, I would love for CSL to support both citation styles. Adding this support to the language, though, requires both deciding exactly how formatting would work (e.g., how should they be coded, how this code should be interpreted by processors, deciding what edge cases need to be supported, identifying the diversity of formatting variations that exist) and updating many existing styles to add the new formatting styles. So, there is a lot of work to do on this, so it will be a long time until full support for both formats will be added.

I think that's actually feasible by keeping the author in the text for all types of citation styles if the options (author-in-text or whatever we'd call it) is toggled, so it would create

Smith (1776) for author-date
Smith [1] for numeric and
Smith{FN} for note based styles

This is a fairly massive change, but actually would be an improvement in terms of switching back and forth to the status quo, where you get
Smith {FN} for the last case.