Modify pnas2009.bst file

I am using pnas2009.bst for my bibliography which I am very happy with but I wish to make a few modifications:

1. I wish that publications with large author lists (say greater than or equal to eight) to be condensed to the first six authors then three ellipses (full stops/periods), and then the last author's name. I know there is a few questions answered on condensing the author lists of citations but the code of a .bst file is so confusing that I think that given this example is different it is worth demonstrating how it works in this case.
2. Book chapter titles are currently currently not printed, they need to be.
3. Webpages do not have their urls after them, they should preceded by "url:"
4. I would like dois to be included at the end of each citation preceded by "doi"

Here is an example:

\documentclass[11pt,a4paper,oldfontcommands]{memoir}
\begin{document}
ABCDEFG\cite{Young2011}
HIJKLMNOP\cite{Alamancos2014}
QRST\cite{Rohatgi2013}
\bibliographystyle{pnas2009}
\bibliography{library}
\end{document}


gives a bibliography that looks like this:

[1] Young, N. D, Debelle, F, Oldroyd, G. E. D, Geurts, R, Cannon, S. B,
[MANY MANY MANY AUTHORS] (2011) The Medicago genome provides insight into  the evolution of rhizobial symbioses. *Nature* 480, 520–4.

[2] Alamancos, G. P, Agirre, E, & Eyras, E. (2014) in *Spliceosomal Pre-mRNA
Splicing Methods Protoc.*, ed. Hertel, K. J. (Humana Press) Vol. 1126, pp.
357–97.

[3] Rohatgi, A. (2013) WebPlotDigitizer.


but i want it to look like this:

[1] Young, N. D, Debell ́e, F, Oldroyd, G. E. D, Geurts, R, Cannon, S. B, Udvardi, M. K ... Roe, B. A. (2011) The Medicago genome provides insight into  the evolution of rhizobial symbioses. *Nature* 480, 520–4. doi: 10.1038/nature10625

[2] Alamancos, G. P, Agirre, E, & Eyras, E. (2014) Methods to study splicing from high-throughput RNA sequencing data, in *Spliceosomal Pre-mRNA
Splicing Methods Protoc.*, ed. Hertel, K. J. (Humana Press) Vol. 1126, pp.
357–97.

[3] Rohatgi, A. (2013) WebPlotDigitizer. url: http://arohatgi.info


Anyone able to help?