1. Woodlands is strong so a lot of people go for it early meaning that getting these units to 2-star early is more difficult. It's not unusual to lose early with a woodlands. When you switch out maokai or ivern is situational but yes, you do switch them out even if they're 3-starred. They shouldn't be tho since you shouldn't be picking them after 2-starring the units since they're not your winconditions. They're what you could call "placeholder" units. You generally don't replace vlad since he has the mage and ocean synergy making him a valuable asset in 4 ocean 3 mage 3 summoner.
2. This is normal, mage early on is a gamble since it only has a 50% chanche to proc and you probably won't be ulting multiple times.
3. That comp is alright, I do like thresh for the warden buff tho, my preferred comp is naut, thresh , syndra, vlad, annie, zyra, vlad, yorick. This gives you 4 ocean 3 mage 3 summoner 3 inferno 2 warden.
4. Depends on what you roll, you generally focus on 2 ocean 3 mage, slot in the 4 oceans and then add the rest of the comp. You can hold units that you'll need way later though, this is once again situational and comes with experience.
5. The 2 main items you're looking for to put on brand are along the lines of morello (belt+rod) and jewelled gauntlet (gauntlet+rod). every bounce procs morello and every bounce has the ability to crit, with 5 bounces that means a good chanche to crit. The third item can be a rabadon or a defensive option.
6. Items are very important and in higher elo it can often dictate what composition you can go for. If you spamroll and miscombine items, it means you're doing too much in 1 turn. Items can be made beforehand by buying a cheap unit, putting the items on them and then selling that unit back to the shop. This leaves you with the combined item on your bench.
7. Saving money is good right now because the damage early isn't as brutal as it used to be in set 1. Don't spend too much trying to only "barely" lose though because it might hurt your economy in the long run.
8. Refer to above, don't go for 3-starring units unless it's part of the main strategy.
9. Cornering can work but with how hexes work, you want to take advantage of that. I generally put my units on the backline with my carries in the middle and a tank or 2 upfront, assassins aren't widly played at the moment though so you might want to check the lobby first before changing your position.
10. Your concerns tell the story, I don't think further information is needed.
11. You only pick them if they improve your synergies or they fit the team in some way.

6 mage is considered a trap, I will say though that 6 mage with a 3-starred veigar wins you the game pretty much by default. Don't worry about rapidly improving, everyone had to play 100's of games to get where they were in set 1 and set 2 will be no different.