1 - there are HUGE different, but there are not highlighted.
Basically, more you progress in terms of combat doctrines more your attack will be bloody. As the name of the doctrine says it "transition to trench warfare" says that you are in a middle-ground between movement and trenches. So your front is still fluid but you should take higher losses when you attack (and particularly if you attack in winter : it gives a malus on attack). If russia already takes heavy loss ot's beacause their morale should be very low (after all you send the whole german army against them) and because ..... their army is composed of bad quality moujiks divisions.
On trench warfare, you have huge defense bonus, and you can have even hgher ones by raising your trench level (there are 3 levels, simulating the degree of complexity of your trench network), by reascherching accurate technologies (trench Mortar A, trench mortar B, Land Mines IRRC).
And.....atacking Russia or France? It's your choice and your strategy general, be aware that in trench warfare, evrything you do can cost you national morale or cost it to the enemy (losses in combat, terrotories gains/loss, objectives city, success/failure in a great offensive, ...)
2- grand offensives is a tool to plan offensive like thar happens IRL (CHemin des Dames, Verdun, Gallicia, Gallipoli). You select at the interphase the objective of your offensive, the armies involved in and the combat techniques you will use and test (combat gas, massive bombardement, flash bombardement, infiltration,... and of course, the date of beggining.
At the end of the year, the game will check if you conquered the objective, if yes, the grandt offenisve is a succes, you will earn a MASSIVE boost of NM (+10) and the ennemy's NM will fall, if not the contrary happens.
You must, in trench warfare, perform one grand offensive per front per year, otherwise, NM penalty for your cowardice (your pepole want you to win the war).
Be aware that the success of a great offensive is different between western and eastern front.
Western Front : take only the objective
Eastern Front : take the objective and five more régions (to simulate the less rigid front in russia)
You say that you suffer heavy loss to take a single region in France, it's exactly that Trench Warfare is. Remeber that thousands of men die for some hill or forest : that's WW1 (ans that's why this game is so fun).
3- I think it's because the AI handle its diplomacy very badly. However, remember that better the war goes for you better your diplomacy will be (all minor countries want to be in the winner's side).
4- Kaiser Wilhelm II is very proud of his dreanought fleet, so you have to request his permission to risk it. In game terms, a die roll evry time you order the HSF to go out of Harbour, if it fails your order is not taken in account (The Kaiser says NO). Having a charismatic admiral in command who can convince the Kaiser may help (an admiral with a special trait - can't remeber the exact name) or selecting the warplan option that gave you a special kaiser permission to send the HSF out.
It's a german specific rule
5- Put fighters with HQ to scout mission, in trench warfare with proper doctrines, bombers can help you and reduce ennemy trenches in combat,-strategic bombings can cripple ennemy economy a little (we are not in 1944 anyway) - aces which reveals themselves in dogfights boosts your NM - use technology to improve your aircrafts and transform them from the useless things of 1914 to an master weapon.