<em>"What is really needed is for the IPCC to more seriously look at 20th century attribution, and this would include looking at a range of volcanic and solar forcing reconstructions."</em>
IPCC assesses the literature that exists. That literature shows that it is hard to say very much that is clear about the the early 20th C because of the uncertainties in solar and aerosol forcing and lack of multiple lines of evidence related to internal variability (no ocean heat content data, poor data availability in southern hemisphere etc.). So I'm puzzled as to why you think something more exciting will emerge in AR5. I'm all for people trying different forcing histories (see Schmidt et al, 2011; 2012) - but it won't change much in this case.
<em>" 'Most' is not a particularly useful concept in context of trying to understand the role of natural vs anthropogenic forcing."</em>
'Most' isn't a concept, it is simply a description that fit (and one that isn't that <a href="http://www.realclimate.org/index.php/archives/2012/01/the-ar4-attribution-statement/" rel="nofollow">obscure</a>). The IPCC statement in question is not the only thing that could have been correctly said, and neither can you conclude that anything not said by IPCC necessarily incorrect.
<em>"And the insistence that pretty much all of the 20th century variability is forced is not useful, with the almost complete dismissal of the possibility of the multidecadal ocean oscillations such as PDO in the attribution argument."</em>
No-one is insisting on this (except Scafetta, but I presume you are not talking about him). Instead, for long-term (ie. multidecadal variations), all of the evidence is that there aren't large (>0.2 deg C) unforced trends in global mean temperature (though regionally there are bigger changes). If you think you have evidence to the contrary, present it. Your evocation of the PDO is strange though - that is <a href="http://jisao.washington.edu/pdo/PDO.latest" rel="nofollow">specifically defined</a> as the departure from the global mean, and so doesn't play a role in defining the global mean changes.
Additionally, during the 1910-1940 period you have highlighted as being important to assess, the <a href="http://data.giss.nasa.gov/cgi-bin/gistemp/do_nmap.py?year_last=2012&month_last=2&sat=4&sst=1&type=trends&mean_gen=0112&year1=1910&year2=1940&base1=1951&base2=1980&radius=1200&pol=reg" rel="nofollow">Pacific temperatures are neutral to cooling</a> (at least in HadSST) - almost all the warming signal is in the Atlantic/Arctic regions.
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