Good post
So according to you, are Ukrainians and Russians the same people - or are they essentially several different cultures/ethnicity, that only happen to share a common related language, due to an interwoven history of war&conquest from 1700 onward to 1990. ?
First of all, I wouldn't begin counting only from 1700s. We shouldn't ignore the old Rus state (though after Yaroslav Mudriy and till the Mongol invasion it wasn't 'a state' , but a group of principalities constantly fighting with each other); the Grand Duches of Lithuania and Moscovy, when each one considered itself a true Rus state and fought for former Rus lands; uniting the GDL and the Polish kingdom into one state, and policy of polonization that gave an impetus of pro-Russian tsardom sentiments.
In other words, it spans centuries and centuries, when people communicated, integrated, resettled, fought, lost and gained lands, etc. When the Ukrainians formed as a nation? Well, in my personal opinion it was quite late, maybe only in the beginning of the 20th century. It is rarely mentioned now, but when the Russian empire fell in 1917 and the Ukrainian People's Republic was declared and Ukrainian Central Rada formed (the parliament), the first decrees it passed were about an autonomy inside of 'a new' Russia, not an independent state.
But then again, I don't think that the Ukrainians (or Malorossy as they were called then) thought themselves as one and the same with so called Velikorossy (the Russians now). Differences in traditions, closing, languages made them different from their 'cousins', in their own eyes. I have read that even intermarriages were quite rare between them. Once again, the word 'moscal' (a derogatory term used by Ukrainians toward Russians) wasn't invented in 1990s or so, but goes centuries back. The same is with 'hohol' and 'katsap'.
But I don't think the case could be called 'nationalism' back then. While the Malorossy considered themselves different - 'non-moskals' - they didn't think in a term of a different nation. Social issues were the main concern then, not national ones. I think it is possible to say about 'regionalism', or something like that.
Now, definitely. The Ukrainians consider themselves as a political nation. Totally different from the Russians, just with similar languages. Moreover, this nationalism is based on 'anti-Russianism'.