1.combat

For running away enemies, they will stop and return to attack, if you follow them you get hit from the enemies not running away, think of it as a tactic to draw you in and don't follow or swing just let them run and return.
With the bow, use leading aim, this works most of the time, aim a short distance ahead of the direction the mob is running.
The same height enemies issue, is that there are animations and this changes the placement of the hitboxes. Using parry and dodge / roll is your best bet here, once staggered they are easier to hit. Also incase you haven't found the recipe yet there is an AOE hammer you can craft early on that will help with the running away enemies and the same height enemies :)

With the enemies strength problem, the game tries to give hints from the very start without too much hand holding and for first time players it might be abit confusing but at the spawn the stones tell players which bosses they should kill and in which order but it can be abit confussing. The order is meadows, black forest, swamp, snow mountains, plains. Make sure to check this order again after major patches as more biomes and bosses are supposed to be added.

2.grind
The "grind" in this game is pretty balanced and alot better than in many other games. As you move past the meadows, you tool strength is better allowing you to collect wood faster, once you have enough ore, making a metal pickaxe makes collecting ores faster. You also unlock a cart for over land hauling which makes collecting resources on your island faster as well as your access to faster ships with cargo space to sea travel is faster. As you progress through each biome these upgrades unlocked for each level making everything faster.
However it is good to remember the game wants you to have adventures, it wants each outing to have the chance to lead to a fun experience, being chased by 2 trolls while pulling a cart, fighting a sea serpent while carrying a haul of ore in a storm or just finding a beautiful view at sunset after finding 5 copper deposits really close to each other.
The destination of a new set of armor and a new shiny sword but the game tries to make sure your journey, from leaving your base to collecting the ore and wood to returning home is filled with adventure and fun and in a monster filled game that fun is alot of battles and close calls and if you die, then hopefully a learning experience of what could i do differently next time.

3. servers
I think you made one mistake here, Valheim is a co-op game and not multiplayer, as such it has the connection interface for a co-op game. I have hosted lan co-op, internet co-op and rented a server for the game and connecting was always really easy except for the rented server where we had to figure out the ip address.
For lan or internet co-op, just start the game and select "start server" after character selection, add a password if you like and then you can invite the people through steam friends list or pm/dm the people your ip and they can connect. hope this helps :)

4 wierd glitches
For the campfires, if you build a half-wall / full wall around them they will stop going out every few seconds, think of it as a windbreak. Also pay close attention to the ground because sometimes the uneven ground will still cause a gap and you will still get the fire going out.
As for the trees needing to be on the same level to chop, here you are right is can be frustrating. I have found that leaving stumps and letting logs roll down to them is the easiest way and the safest, the trees in this game are as dangerous as the mobs and i have been killed by falling trees more times than i am comfortable admitting =D

Hope this helps and hope you continue to have an awesome time ingame.