WELCOME! 

1) depends on your device, now with iftt/hass you can integrate and use both.

2) try to avoid broadlink switch since they don't last long. There's also chuguan brand ( i have this one and still working since last year) and other rf switch you can find at taobao

3) both compatible now there's alexa & google home skills.

What so confuse ?

Just use Xiaomi la.

This is correct.
What problems you want to solve with using aqara or wifi switches?
Alexa and Google Assistant are just a minor part in the smart home. Can run from your android phone app for testing before u buy the smart speakers.

You already mentioned it earlier.
You want to change your dumb switch for Aqara switch to do what? Be another switch? 

Nope, not offically sold here nor comes with msia zwave frequency. If you wanna get one, make sure your hub z-wave frequency is compatible with your z wave devices.
For example, I got a us smartthings hub to pair with my Yale smart lock.

The thing with my automation is it only suits me. My habits and my home is unique. I dun see how that can help you.
Nevertheless I can tell you what I do with mine.

1. I set my front porch lights to come on at certain time on a daily routine.
2. When I open my front door, the door sensor will trigger certain lights to come on like my living room lights and dining, etc.
3. I use motion sensors to turn on some lights and off after a certain period of time
4. I set one switch at my front door and another switch at my bedroom door to turn off all lights in the house when I go out or go to bed. This is the only time I need to touch the physical switch
5. I got water leak sensors to warn me of any leaks or rain water had enter my bedroom windows
6. I got smoke alarms as well. If it detect smoke, it will turn on all the lights. Sound the alarm of course
7. I got temperature and humidity sensors to warn me say my camera box drops below a certain RH
8. When someone rings the doorbell, it will send me a notification to my phone. I can use the ip camera to see who it is.
9. All the door/ window sensors and motion sensors can trigger an alarm if I armed it before leaving the house
10. My bedroom door sensor will turn on certain lights when it’s open at certain time. This way I dun have to walk into a dark living room.
11. I placed some window sensors on some windows to sound a chime and light up the gateway if they are open.
12. I set my air filter to turn on if it detects a reading of over 50 and off when it reduces to 15

Here are my Aqara stuff. Some not install yet though

Yup using them. No issues so far. About 10 months using now

Any links to the forum?
What kind of problems?

this is not what i meant.... I have a wall switch given by the developer which is a momentary switch that has been wired to control my auto gate... i just plan to replace that wall switch with a sonoff switch... the only difference is that in a momentary switch, when you press it will complete the circuit... but when you let go it will go back to its original position.... so it just like pass the current for a short while... was womndering how to do it for smart switch

i also want to buy 3rd gen but shipping cost still too much.

get from bhphotovideo, shipped for a pair around RM 550

where from? which 3rd party?

Yup, can do that in Home Assistant and that will be what I set if I have an auto gate.
Plug in a Wemos D1 Mini programmed as a binary sensor and plugged in to your car.
When your car starts, the binary sensor connects to your home wifi and becomes connected.
This triggers the auto gate to open through automation.
When your car leave, the binary sensor will disconnect from your home wifi and it will be disconnected.
This will trigger the auto gate to close. Again through automation.
Not really. Yaml is simple. Just that need to get the spacing right.
Got 2 ways of using Alexa through HA:
1) Home Assistant Cloud - if you can afford Starbucks every week, it is not too expensive to subscribe to the HA Cloud. No need to set DNS and port forwarding.
2) Use the free compenents as per this LINK. More on creating the skill in Alexa and the yaml side.