All wireman/eletrician can do it. the price will wary based on workmanship and distance.

It is either the sensor, or the communication, from sensor to hub than hub to device. Easiest way to check the log from the hub

Hmm... my battery last more than 6 months.... the scale only turns on when someone stands on it. and I use it once daily.
Basically it does not consume much power.

Maybe can do a bit of experiment, Buy new batteries pack consist more than needed by the weighing device. Then measure the battery voltage for those that is put into the device and those that is not use as standby.... and continue measuring for few days...

Then from here can know better the battery is really drain from the device and what is the rate.

Price range varies with regions and also distance from circuit box. Generally price is similar or slightly cheaper than price of pulling new wires. RM50~RM150
If you are only asking to do one switch, the price will be expensive.... but if you are doing the whole house...it will be much cheaper.

Besides, you do not need neutral line to all switches... 1 per cluster will do. Switches normally gather at 1 side as 1 cluseter, especially for bedroom. So 1 neutral will do.

Do you mean install 2 smart-locks? one on the grill door and then another one on the inner wooden door?
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>>also will it be weird to combo it with smart lock for the wooden door?
It is not weird but it just show that you are rich
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>>it is possible to combine both of them?
>>eg: unlock the drill door will auto unlock the wooden door?
My poor understanding is that if you only want to use the lock once? why install 2 locks?

I do understand some house usually in older area have this grill things... even for the windows.

Technically
It all depend on the features of smartlock but from what I know, if the smartlock support remote unlock through hub, then there should be no problem to link the 2 smartlocks.

But actually my question previously is more of a conceptual/opinion discussion
>>My poor understanding is that if you only want to use the lock once? why install 2 locks?

If you have 2 or multiple doors that unlock automatically after 1 another, is there added security?
One may argue that what if some guy break-in and 2nd door is a buffer.... but your first lock and alarm system should provide defense against it at the first place.
and if you uses same token, card system, passcode to unlock both smart lock or automatically unlock from status of 1st smart lock, then the additional smart lock is of no security use theoretically.

I answer some that I know... maybe others sifu and commends.
Aqara is quite OK brand, there is also Tuya.

Aircond switch is abit unique, because for aircond, even if you switch on, you still need remote control to switch on. That is why there is IR blaster remote.
or you can directly consider new aircond model with network capabilities built in

if you use zigbee, you need a hub... the switch communicate through zigbee to hub... and hub communicate to internet. Zigbee advantages is that, it does not overload your wifi router.

compatibility is some thing good.... If it only support mihome, all your device, switches will need to be using the mi environment. It is best that a device can support various environment and it will be easier to setup.... that just my 2 cents...

If your aircond already have smart with wifi control, don't bother to use wall switch.
the standby current of your aircond should be the same as for the smart switch. I don't see any purpose to install additional control for the air cond.

as for tmall genie, no comment as I never use before

Haha... red doesn't necessary mean Live though... haha... our local electrician never follow wire colour code.
Best is to check with a test pen

Wow... 10 years ago already pull neutral wire? You sure it is not looping wire which is no longer in use?