Thanks Rasmus and Eric!

1) subtitle

The git version has a #+subtitle keyword.  See the git version of the
manual for supported backends (most).

Where the git install instructions say "you should edit local.mk to point to the appropriate install location", how do I find out where this is? Sorry, not used to using git :/

Is it possible to install the git version alongside the stable one, and choose which to use for each session?

2) level formating

For LaTeX export, this will not work (AFAIK) as LaTeX only supports 4 or
so levels of headings.

Just tested: yes, 5 levels. At the 6th it treats them as list items. Well, it may be sufficient - I put '9' to leave some room.

Depends on the backend.  With latex you could use setkomafont (of
KOMA-Script) and with html you could use css.

If you want this kind of control, you should ensure that the LaTeX uses
the scrartcl (koma-script) article class which is highly
configurable.  However, you will have to do the LaTeX configuration
yourself directly using #+latex: and #+latex_header: org directives in
your org document.

For HTML export, you will need to define the appropriate CSS
specifications to achieve the stair case effect.  I have no idea how
this would be done...

Right, I'll have a look at koma-script. Would there be a tutorial/example somewhere for how to do this from Org?

And perhaps this can take care of the subtitle issue, too, within the stable Org version?

3) Custom highlight markers error

No idea about this.  It could be a bug anywhere.