What do you want to see as a "proof in practice"?
SU had all the nececery equipment to make that happen(over 26k tanks, 7k fully functional), planes, mechanised corpses, infantry and military exercises.
But SU was cought before it`s army was mobilised and most of border troops were destroyes. First succesfull Soviet deep operation would be battle of Stalingrad.
Wrong. Battle of Stalingrad happened as it happened only due to Hitler being an incompetent micromanaging supreme commander. 6th army wouldn't have been caught with their pants down with anyone competent in command. Moreover, it would have broken out with anyone competent in command. Even with Hitler complicating things, germans got pretty close to getting a connection to the surrounded army.
Without Hitler, things would have been completely different. The relief operation could have been started earlier with more adequate forces, and even from within the pocket. Then again, without Hitler there would have been no pocket, and perhaps not even the whole war. As we know it at least.
This has nothing to do with "deep operations". It was a matter of simple pincer movement through weak front sectors (romanians, italians). Nothing fancy, just exploiting weaknesses and applying enough force on the right spot.
It is a succesfull display of capability of Soviet army to fight in complicated terrain. The cassualty rate is quite normal for conditions.
Mind you in WW1, UK+France had to loose 3:1 for each German, and they were fighting good terrain, with good logistics and good weather. Soviets fought on a narrower front, Fins had a decent defencive line, and better readiness to fight under weather conditions.
Yes, Soviets didn`t expect Fins to have the spirit to fight. But in the long run that was irrelevant, as Soviet army achieved what it wanted, territories to better protect Leningrad and Finish uncapability/unwilingness to advance far into Russia WW2. But which of world leaders ever envisioned military campaign right? Allies, Soviets and Germans all "envisioned" their troops overperform.
As for books&stuff, Please, provide better arguments that dull acusations.
Finns had a decent defensive line? WW1 front between France and Germany had good terrain? Soviet troops had never experienced winter? Soviet leaders wanted only to protect Leningrad? Oh wow, where to start... At least you so far haven't blamed the Winter War on the finns...
The main line (only one which actually could be called fortified) was 140km long. Per kilometer it had 4 field fortified machine gun nests, 2,5 km of barbed wire and 1 km of anti-tank obstacles. The line had 101 concrete bunkers. Natural obstacles were of course utilized, but as you stated yourself, the winter negated whatever hindrance swamps and lakes and rivers would offer. Adding the snow of course.
The fact that the soviet offensive was planned so badly and the leadership under estimated the defenders does not negate the fact that the Red Army pretty much fell on it's face on the Karelian Isthmus. Of course as a result soviets had to come up with excuses as to why this has happened. The Mannerheim line was made to look like the Maginot line of the north in the eyes of the public. Of course, the fault of that assumption is that it's completely ludicrous. Maginot line had more than 50 times the amount of concrete bunkers on the same length of terrain in comparison.
The weather argument is just silly. Soviet Union was after all just across the border from Finland to the east. They have exactly same sort of winter and weather conditions there. Just because one crosses the border does not magically change the climate. The Red Army had had troops stationed all the way to the Kola peninsula for years, so they pretty much knew the conditions to be expected for a winter campaign.
It's also a bit odd to call WW1 west front conditions good terrain. There were frontline fortifications all along the front. Trenches in depth, great forts like Verdun even. The no mans land was shot to pieces by both sides god knows how many times over.
If the soviets only had wanted to protect Leningrad, why had they prepared to have a victory parade in Helsinki? Why had they taken whole Finland as a part of their sphere of influence in Molotov-Ribbentrop pact? They gobbled up all the Baltic states and half of Poland but only really wanted to secure Leningrad and only wanted the Karelian Isthmus from Finland? Does not really seem likely.
Soviet Union and their Red Army just had met no real opposition so far. They had unrealistic expectations and got a bloody nose in their effort. And as a result of this humiliating display of "power" and "capability" they did start reforming their purge riddled forces.
I do admit that finnish army was in a pretty bad state by the end of Winter War. But Stalin lost his nerve. The western Allies might have intervened eventually, especially with all the activity going regarding Norway.
In the end, I still do think you have fallen victim of soviet/russian history books. Seeing what is going on around there even today, I would not trust their interpretations of any events without having it confirmed from an independent source.