Probably you would want to consider using the changepage
package. With it you can locally adjust the margins of text in a page with the use of the environment, adjustwidth
.
\begin{adjustwidth}{<leftmargin>}{<rightmargin>}
<content>
\end{adjustwidth}

\documentclass{article}
\usepackage{changepage}
\usepackage{lipsum}
\begin{document}
\lipsum[1-2]
\begin{adjustwidth}{2.5cm}{} % Here 2.5cm defines the extra tabbing you need. The right margin is left untouched to keep the default right margin.
\lipsum[1-2]
\end{adjustwidth}
\lipsum[1-2]
\end{document}
For the second question, you can use a list as David Carlisle suggested. This can be achieved as follows:
\documentclass{article}
\usepackage{changepage}
\usepackage{enumitem}
\begin{document}
\begin{adjustwidth}{2.5cm}{}
This paragraph runs to the proper end of the page. It will have a fair bit of space at the end, unlike the next paragraph which will overshoot it because of the makebox that's put in front of it.
\begin{enumerate}[label=1.\arabic*,leftmargin=*]
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\end{enumerate}
\end{adjustwidth}
\end{document}

Now playing with lists to make more robust:
\documentclass{article}
\usepackage{changepage}
\usepackage{enumitem}
\newlist{parin}{enumerate}{2} % depth of 2 can be changed depending on the necessity
\setlist[parin,1]{label*=1.\arabic*.,leftmargin=*}
\setlist[parin,2]{label*=\arabic*.,leftmargin=*}
\begin{document}
\begin{adjustwidth}{2.5cm}{}
This paragraph runs to the proper end of the page. It will have a fair bit of space at the end, unlike the next paragraph which will overshoot it because of the makebox that's put in front of it.
\begin{parin}
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\begin{parin}
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\end{parin}
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\begin{parin}
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\item I have a whole bunch of text here and it will extend past the edge of the page even though I'd like it to break before the page ends and act like a normal paragraph.
\end{parin}
\end{parin}
\end{adjustwidth}
\end{document}

tabularx
or (usually better) a list environment probably does what you need. Please always include a complete small document that shows the problem.\makebox[2em]{1.1}\parbox[t]{\linewidth}
is clearly 2em too wide but again you should be using a list environment with\item[1.1]
not parboxes.item
fromparbox
does give me something that's much closer to what I'm looking for. I think it's safe to say that last bit solves the problem. Thanks!