The OpenRussia site, run by Khodorkovsky as far as I know, has published the full report which was being prepared by Nemtsov before his death, titled "Putin. War." (in Russian) here:
https://openrussia.org/post/view/4803
Some of the main highlights:
-Firstly, it lays the blame chiefly on Putin and his staff for the war, suggesting that the main motivation for escalating the conflict has been electoral politics. The goal is to secure Putin's power base by effectively radicalizing most of the Russian population around a new nationalist agenda. It suggests that the media has been instrumental in setting the agenda - and that the consolidation of media under government control in Russia over the last couple of years is related to this.
-Secondly, it reports that the Russian military personnel involved in the takeover of Crimea had been briefed and dispatched from the start of February 2014, preceding the violent turn of events on the Maidan in Kiev. That is to say, the report alleges that Russia's annexation was not a reaction to the violence in the Ukraine, but was executed at a pre-planned time.
-Thirdly, it suggests that Russian forces had saved the separatists in the Donetsk region by intervening directly in August 2014 - up until then, the Ukrainian army had been making significant progress in their advance on the separatists, whose efforts were collapsing. The report alleges that this rapid intervention was decisive, and was the only way that the separatist structure has survived. The report also notes that the majority of current separatist leadership, which has changed quite a bit since its early days, are Russian citizens. It thus concludes that the separatist side is effectively under Russian control, and has been since at least fall 2014. It further alleges that all key decisions and information releases by rebel leadership are managed directly by the Kremlin.
The report goes on to cite numerous interviews with separatist and Russian officials, as well as stories of soldiers captured on Ukrainian territory.
-Fourthly, the report alleges that the so-called "volunteers" fighting on the separatist side are better called "mercenaries", and are paid $1000-1500 per month. This is in excess of the average Russian monthly wage of approximately $620. The report suggests that this money comes directly from the Russian federal budget.
-Fifthly (and I expected this one!), the report details a significant role played by Kadyrovites (i.e. militia controlled by the Chechen president Ramzan Kadyrov) in sustaining the separatist effort. This probably has significant implications in Nemtsov's murder, which has been blamed on Kadyrov (to nobody's surprise). Unlike other high-ranking members of the current Russian regime, Kadyrov has publicly admitted sending armed fighters to the Donbass. The report notes that he has issued lavish awards to many members of his security forces for their participation in the Ukrainian conflict.
-Sixthly, the report looks into the shooting down of MH17, and agrees with findings that it was shot down by the Buk M1. It also points to the previously-suggested launch site from the village of Torez, which was under separatist control at the time of the shooting.
Finally, it also looks at the economic and human costs of the conflict, which were mentioned before.