Boooo, peaceful solutions make boring history.
I would have told Douglas Haig to listen to Churchill and the French. He deployed the first version of a tank onto the battlefield before he had enough to make a significant impact. A large offensive with the use of tanks could have broken the stalemate along large chunks of the line, possibly ending the war sooner.
For the Germans, I would say the same about their use of mustard gas. Should have stockpiled and used it all in one go.
Still would have been the best outcome for Ger. They would probably have gained territory instead of losing so much.
I seem to remember tanks where of little use versus heavily entrenched terrain. Especially since many frontlines where rivercrossings.
WW1 tanks were terrible, and they got stuck all the time and broke down. But it was way better to have them than not have them, because they got through barbed wire and gave a small piece of cover for infantry. Tanks were partially responsible for some of our biggest gains in the war along certain parts of the front, not that it mattered much in the end. I would rather be sent into no man's land than try to get one of those things across a river.
The Lenin thing is great, but it also backfired in a big way. Communism started to spread to German soldiers on the eastern front while all the Lenin stuff was going on in Russia. When the Russians left the war, those soldiers were reassigned to the western front and it spread there too, which caused a big dip morale and the willingness to keep on fighting.
I would tell German Chancellor Theobald von Bethmann-Hollweg, Paul von Hindenburg and Erich Ludendorff to listen to General Erich von Falkenhayn on 18 November 1914. When he proposed to pursue a diplomatic solution, after seeing the effects of the first battle of Ypres.
https://en.wikipedia.org/wiki/Western_Front_(World_War_I)#1914.E2.80.94German_invasion_of_France_and_Belgium