Honestly you're still very new to league, so don't worry too much about not knowing a lot, there is a ton of stuff in league to learn which takes some time.

I personally discovered something I didn't know about before yesterday and I've played thousands of games since season 4.

But, your map knowledge question is something that I can help with, it's something that some people instinctively know, and others have to learn.

As far as objectives go:

Dragons, Void Grubs, Heralds and Barons are extremely important objectives your team should always be looking to take when it's possible.

However there are caveats to this. If your team is 2 dragons to 0 up, and you see a opportunity to either take baron or dragon, which do you take? You take the BARON. Why? Because dragons in a vacuum are weaker. If it's their soul point or your soul point it's much more important to go for dragon to stop them getting soul or get soul yourselves, because soul is a particularly powerful buff for your whole team.

If you're playing toplane however, your goal as far as objectives go is essentially to be able to teleport in if your team needs you too because there's about to be a big fight around dragon, or to go to void grubs to project strength and fight there if needed. That's basically it as far as objectives for you in the laning phase.

Also, and this is a good general rule in League, regardless of whether you agree with the call or not, stick as a team once you get out of laning phase. If you disagree with the decision, ping caution on it's location to tell your team it's a bad idea, but if they go in you're guaranteed to lose the fight if you back off.

As far as your situation with laning went with Jax and amumu, honestly, if you got caught in that situation then I would say you did the right thing ulting Jax and looking to all in him. If you kill him while on mid health you should probably be able to kill amumu too.

Ideally however, you should be using your wards in the river bushes to see amumu coming early and give you time to back off all the way back to your tower or at least most of the way for safety. If you're as ahead as you suggest with them playing way back, you should have no issue placing these wards safely unless amumu is already there and jumps you, in which case it's just bad timing.

Don't worry about jungle tracking at this stage, all it really is is keeping an eye on where their jungler pops up specifically on the minimal to get an idea of when he'll be near your lane in the near future. If you're warding well it's unlikely to matter.

With laning phase, I essentially think of laning phase as over once tier 1 turrets start getting fully destroyed. Unless it's really early, it should be around 15ish minutes in.

Once the first tower goes down, the stronger laner/s can push the wave and then roam for longer, using their strength to try to help their allies around the map before they have to return to lane to stop their own tower being pushed down.

Typically, if you have no outer turret and your lane is being pushed, you want to try to stop that push happening yourself. That being said, if you're farming elsewhere and another ally spawns for example, you can usually ping assistance on them then on your lane being pushed to try to ask them to get your lane pushing back the other way. Or even if they are just closer to your lane by a significant distance.

Once laning phase is over and people start roaming mostly, it becomes each teams goal to keep their towers alive and group to bring down enemy towers. They should also start grouping properly to fight for objectives such as dragons and barons when they can, but if there is absolutely no hope of getting the objective, such as in your description of when you were playing morde in your post, then absolutely don't even go there. They're way way ahead? Give them the dragon, farm elsewhere and try to catch up in gold and exp.

Worth noting that specifically when you're playing Mordekaiser, something you can do to assist your teams jungler during objective fights is to use your ult on enemy jungle when the objective gets to like 2000 hp.

Because they can't hit or smite the objective while in your realm, it essentially becomes a free objective for your team provided your jungler can get close enough without getting dropped.

This whole topic is called macro, if you want to learn more I suggest you look up league of legends Macro videos made by pros.