1. Powerline is dependent on your electrical wiring. If you have crappy wiring or a lot of interference (microwave, AC, high speed motors), you'll have crappy connection. Newer powerline also use the ground wire and if you don't have that, then you'll lose some of the speed benefits. The further your adapters are from each other, the slower they are. Outlets close to each other may have 100ft (30m) backhaul between them. You'll never know without getting behind the walls. You should test the optimal performance by plugging both powerlines into the same outlet. Don't use surge protectors or UPS. You can use a short passive extension cord to plug them both the same outlet, but cord quality will affect signal quality. Since you're in an apartment, you should enable encryption on the signal, but (just like wifi) you'll lose performance.

2. Yes it works like mesh WIFI. The "amplifier" shouldn't reduce ethernet or wifi speed. It's an extender, not repeater. But don't expect wifi performance from it to extend far from the room. It's a small extender without real antennas.

3. I'm in the US, but I've tested multiple cheap USB NIC's and they've all worked with my Mibox.

I have Zytel PLA5206 and with encryption and optimal performance I get 25MBps (200mbps). Performance in rooms on the opposite side of my house (multiple walls are between them.) is around 10MBps. My Wifi with encryption is around 30MBps, but not as consistent. Wifi on opposite side of my house is under 10MBps and inconsistent.

Powerline has a lot of variables that can reduce signal quality, but for a lot of people (like you), it solves the issue. Another alternative is to use flat or slimrun/micro (Not sure if they're available in your country) network cables and run it along the walls. Or run ethernet as far as you can and then use a WIFI access point.

For long distances with obstructions: hardwire > WIFI access point with ethernet backhaul > Powerline > WIFI repeater > WIFI only

You can also look at MoCA if there is a coax cable available in the place you want to use a wired connection. I wasn't able to run a ethernet cable to a room inside my home but luckily there was a coax cable in that room. With the MoCA 2.5 adapters from GoCoax i can achieve a symmetrical 1 gigabit connection. Coax is much less prone to interference than Wi-Fi or Powerline.

So, I'm not a huge fan of powerline adapter since they are more prone to interference and the quality of the connection depends on the electrical wiring of your house. Also note that the powerline adapters out there are never gonna give you a 1 gigabit connection, even if there is a gigabit ethernet port on it.

fl3abag is mostly right with his post, except for forgetting MoCA in his list.

For long distances with obstructions: hardwire > MoCA -> WIFI access point with ethernet backhaul > Powerline > WIFI repeater > WIFI only

Powerline Adapter are slow as fuck. A normal wifi signal is good enough I guess. It works for me.