WolfBear
Just to say I was unwell for a while and took a week off the site then only really checked the 1st page when I came back so missed this until now.
1) It might depend on events. In the short term the best chance of a republic might be what you suggested a couple of posts back. After a long and bloody but ultimately successful war there will be pressure for social change and more rights for ordinary people. If the military seek to keep control and are sending forces all over the place to maintain their spheres of control, especially with a lot of soldiers dying then you could see a violent reaction. There would be a danger of that ending up as an open dictatorship, if either the hard left won or the military and aristocracy still came out on top but it could end up with more moderate elements in control.
2) They could do if they decide they don't need to bother with Russia having hived off the Baltics, Ukraine, Caucasus and possibly some other areas. Working on the basis their less of a threat than the white's winning. In that case and with no aid reaching the Whites other than possibly Japan - Germany being unwilling to and Britain lacking resources and access then the Reds are likely to win.
3) Impossible to say depending on the circumstances. OTL it was drawing a lot of Poles into industrial areas, although they faced some discrimination. Could draw in people from elsewhere or rely on their own population. There was relatively little non-European migration to European states - and not a massive internal movement disasters aside - prior to 1945 onward. Would depend on how Germany, Europe and the world developed.