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.
> Could you maybe post a small example file?  I have no idea what you are
> describing unfortunately.
I retrieved what I did, from here 
<http://stackoverflow.com/questions/20016634/highlight-selected-text-in-emacs-org-mode>: 
"You can re-define the characters for emphasizing via Options->Customize 
Emacs->Specific Option then input org-emphasis-alist."

Then I added an item to the list for using the interpunct character ·, 
resulting in

[INS] [DEL] List:
             Marker character: *
             Choice: [Value Menu] Font-lock-face: (sample) bold
             [ ] verbatim
[INS] [DEL] List:
             Marker character: /
             Choice: [Value Menu] Font-lock-face: (sample) italic
             [ ] verbatim
[INS] [DEL] List:
             Marker character: _
             Choice: [Value Menu] Font-lock-face: (sample) underline
             [ ] verbatim
[INS] [DEL] List:
             Marker character: =
             Choice: [Value Menu] Font-lock-face: (sample) org-verbatim
             [X] verbatim
[INS] [DEL] List:
             Marker character: ~
             Choice: [Value Menu] Font-lock-face: (sample) org-code
             [X] verbatim
[INS] [DEL] List:
             Marker character: +
             Choice: [Value Menu] Face property list:
                         [INS] [DEL] Key: :strike-through
                         Value: t
[INS]
             [ ] verbatim
*[INS] [DEL] List: **
**            Marker character: · **
**            Choice: [Value Menu] Font-lock-face: (sample) highlight **
**            [X] verbatim*

Then when I write something like ·this· in my text, it is highlighted 
with a yellow background, within Org. But upon exporting I get the error 
mentioned. It does not appear to be due to some interaction with other 
stuff in my document, because the error occurs even for a minimal 
document that has only

·test·

in it. So, a bug as Rasmus suggests, or a matter of configuration?

cheers