1. No Idea, was arrows and stars in DD1 but havn't seen anything in DDA
2. On Insane and harder difficulties the timer has always started instantly. The same for DD1 and you can ot take the timer away. The best way to hande it is to increase your heroes movement and casting speed.
3. No idea. Havn't tried multiplayer yet since i'm only playing solo or with friends. Might be possible to see the stats if you enter the menu and clicking the profile of your party members but probably not.
4. As fas as i have seen it doesn't work. Only sells everything on the map as far as i have seen and it is kinda anoying but then again auto-loot is a new function for DD2 and DDA.
5. When playing Solo you kinda need all the heroes.
Squire: good barricade and physical dmg towers and alot of in line aoe with harpoon towers, doesn't need any kind of resistance drain.
Apprentice: good elemental and burst dmg towers. Needs the use of Magic Missle tower and Deadly Strike Tower alot since the only resistance drain you have is from Magic Barricades and it's not so good as a barricade.
Huntress: Poison trap is great to group enemies up and delay them. Explosive trap is great for a burst of dmg, really good with more stats since it will recharge faster and long range. Inferno traps are good but it's quite a bit of a down time since it has longer recharge time then explosive traps. Darkness trap is needed on higher difficulty maps with alot of cornerns since it makes it so that ranged enemies have to go in to melee it also makes it so that your enemies looses elemental resistances. that means they make it so that enemies can't target towers and heroes through them. Etherial trap does alot of dmg in a burst but it costs too much to be worth it and it needs a Electrical resistance drain to be used.
Monk: All auras has their uses. Slow Aura slows, but need something to get ridd of the elemental resistance. Elecricity aura does alot of dmg inside the area of effect, needs a way to get ridd of the elemental resistance to be usefull, great together with Slow Aura and Streangth drain. Healing Aura heals your heroes, great against boss fights where you need hero dmg. Streangth drain is needed overall since it takes away Elemental resistances and reducess the enemies stats while inside it. It's the most needed aura of them all. Enrage Aura is not really needed. It makes enemies hit each other. Problem with it is that it costs a bit too much to really be usefull since itself does't do dmg and you need atleast two enemies inside the Aura for it to work. else the mobs will just walk through it and you need Strength drain or something else that gets ridd of the elemental resistance for it to work.
Series Ev: Got a Blocking Field that makes it so that enemies can't pass through it without destroying it, it doesn't taunt like the other barricades. Great for blocking enemies before they start hitting it. Low cost DU cost. Reflect Field great for blocking ogre and archers range attacks. Reflect them back against the enemies. Is really good on maps where the mobs can shoot your core from a weird possition were the towers can't shoot against the mobs. Proton beam does alot of physical dmg if the mobs stand inside the area of effect. Not so good later on but has a low DU cost and can be used infront of barricades. Shock beam does alot of dmg and stuns enemies with a low DU cost. Needs a resistance reducing effect to work. Overclock beam buffs the towers, beams, barricades and auras it touches. Is needed on later difficulties since it's a great boost for your defenses in all the stats. Each of the beams has an increase of DU cost the longer you drag the beams.
For solo play you kinda need all the Heroes and you should especialy have two monks since it is really good to have a tower monk for the auras and also a hero monk for the Tower and Hero buffs they can make durring combat. It's the hero that has the most uses of them all since the tower and hero buffs works on all the allies towers and heroes since it's in an aura. One thing that is different compared to Dungeon Defenders 1 is that the Monk Aura for boosting towers is around him and does not move with him.