First of all, I looked
here for a comparison of the various war casualty figures you quoted with those of other countries over roughly the same period.
While the figures do tally with the ones quoted here, the estimates do vary quite wildly, and the quoted figures in this thread almost invariably go with the highest estimate.
A similar comparions ot other countries' death tolls quickly reveals that America's only conflict that qualifies as especially bloody in international terms is the Civil War. This is understandable, since, aside from the Revolution itself, the Civil War is the only conflict America has engaged in that could be seen as a war of national survival, comparable to the multiple invasions and prolonged wars.
Indeed, in the "(Possibly) the Twenty (or so) Worst Things People Have Done to Each Other" table the only thing that happened that resulted in a majority of the deaths being in North America was the "Annihilation of the American Indians".
My point is that while America has certainly had a lot of war dead over the years since independence, her experience has been relatively happy compared to many other nations of note, in both proportional and absolute death tolls.
While it doesn't and shouldn't diminish the respect or gratitude for the fallen, America can also be grateful that her history has been such that there are not a great deal more fallen to be thankful and respectful of. (Compared to, say, Russia, whose casualties are at least an order of magnitude larger than the USA's over the same timeframe.)
Is it possible to sustain a long term (3-5 years) war in the present day?Again, this is filtered through American experience. 3-5 years is quite a
short war in international terms, not just historically (Europe's wars have had names like "The Hundred Years' War" and "The Thirty Years War"!), but also compared to ongoing or recent conflicts (in, say, Congo, Sudan, East Timor)
That said, I don't think any sophisticated modern democracy will easily be able to sustain a single conflict for more than, say, two terms of political office. It differs according to country, but this means something like 8-10 years as a maximum.
Given the mass loss of life that occurred during the Civil War could Lincoln have been reelected into today’s instant media?No, probably not, for the reasons you've already outlined.
But then, Lincoln was gangly-looking and, for his time, not just handsomely tall but freakishly tall. Plus he had thinning hair and a beard. In today's television-driven politics, it's quite doubtful he'd ever have been elected in the first place.
Does the advancement of technology and instant communication mean the end of large scale, long-term conflicts?Not automatically. Driven by hard experience in Vietnam, US forces have quite effectively been able to control the media in subsequent conflicts (through the practice of embedding journalists) so very little that might undermine their chosen methods of conflict becomes public. Scandals do break eventually, but not often until months or even years after the event.
I think the bigger factor undermining large-scale, long-term conflicts is public unwillingness to suffer the consequences.
The American public has never really had to face large numbers of domestic civilian casualties at home, and having seen what can happen elsewhere (probably most notably in WW2 Europe), sensibly doesn't want to have to face such things. (Who said Americans don't learn form history?

)
And they don't want to see their friends and relatives come home in body bags.
And - often underplayed - they don't want to pay for such a conflict, in direct & indrect taxation, but also in the type of national war-footing that pretty much necessitates government control over large sections of the economy.
And in case anyone thinks I've been talking about American publics specifically because I think European publics are very different - I don't think that. If anything, the direct experience of warfare over the centuries, and particularly two world wars sparking off in Europe, makes European publics rather less inclined to war-mongering than even the American public. I think the main - maybe even the only - difference, once engaged in war, is a slightly greater acceptance of deaths on "our side" than the American public is prepared to live with.