Unexpected interaction between biber and a special form of the given-name component of an author's "full" name

With Biber you can use the 'extended name format'. There you can specify the abbreviated form of a name part explicitly if you don't like default.

You would use

author  = {family={Newton}, given={Sir Isaac}, given-i={Sir I}},

And would get

Sir Isaac Newton

in full, or

Sir I. Newton

in abbreviated form.

\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@misc{IsaacNewton,
  author  = {family={Newton}, given={Sir Isaac}, given-i={Sir I}},
  title   = {Opticks},
  year    = 1730,
  edition = {4},
}
@misc{xyz,
  author = {Zoe Zwicky},
  title  = {Thoughts},
  year   = 3001,
}
\end{filecontents*}

\documentclass{article}
\usepackage[giveninits=true,backend=biber]{biblatex}
\addbibresource{\jobname.bib}

\begin{document}
\nocite{*}
\printbibliography
\end{document}

Prompted by comments by @moewe below his great answer -- and especially by the allusion to how biber treats material in so-called "brace groups" (any material encased in an extra pair of curly braces) -- I've come up with another working solution. It's not as thorough as the one given by @moewe, but it's quite simple, in that it requires the use of just one extra brace group to help biber figure out what's the given-name component of the full name. Specifically, all that needs to be done is to replace

author = "{\relax Sir I}saac Newton", 

with

author = "{{\relax Sir I}saac} Newton", 

With the extra brace group in place, biber does not get around to inserting a \bibnamedelimb directive either between \relax and Sir or between "Sir" and the Initial "I". As a result, ordinary interword spaces get inserted. I suppose this could cause a new issue if, for some reason, \bibnamedelimb were to be set to something that's materially different from \space; fortunately, this doesn't appear to be the case here.

A full MWE:

\RequirePackage{filecontents}
\begin{filecontents*}{mybib.bib}
@misc{SIN0, 
  author = "{\relax Sir I}saac Newton", 
  title  = "Opticks", note = "Not correct"
}
@misc{SIN1,
  author = {family={Newton}, given={Sir Isaac}, given-i={Sir I}},
  title  = {Opticks}, note = "Correct"
}
@misc{SIN2, 
  author = "{{\relax Sir I}saac} Newton", 
  title  = "Opticks", note = "Also correct"
}
\end{filecontents*}

\documentclass{article}
\usepackage[giveninits=false,backend=biber]{biblatex}
\addbibresource{mybib.bib}

\begin{document}
\nocite{*}
\printbibliography
\end{document}