External Hard Drive to Smartphone without additional power supply
A) Is my goal really achievable? How?


Unsure, see the answer to the second question of item E.


  B) Is my assumption right, that my LaCie HDD won't spin because of power supply issues?


Yes. The HDD isn't spinning up because the power supply isn't sufficient.

Another reason would be the HDD being broken otherwise, but this is unlikely the case given your very detailed description of your problem.


  C) Will the mentioned WD HDD do the job? If no, other suggestions, please.


For the same reason as B, this is unlikely. 2.5" HDDs nowadays have a fairly similar power consumption, regardless of brand. Older HDDs and HDDs with higher capacity tend to draw more power, but not making a huge difference.


  D) How do I have to format my drives to be read by Android? (I've read ExFAT should work...)


Nothing. You're done it correctly - exFAT is the right filesystem for them.


  E) What are the actual power requirements (in watt) of those drives?


As given in whatever you've read. Those claimed power requirements are usually correct.


  What is the actual power output of my Smartphone over OTG? Maybe someone with experience (and/or who has these drives) knows those numbers?


So, this is the most tricky part...

I haven't used any Huawei phones, but from my past experiences phones don't output much power (usually less than 2W) to OTG devices. It's likely that your phone is doing the same - not supplying as much power as needed by the HDD.

From previous news (in my local language Chinese - sorry), Mate 20 / Mate 20 Pro is capable of running as a power bank, or "reverse charging". I'm guessing that the claimed 5V/1A output is only achievable in reverse charging, not OTG.

The conclusion is not quite delighting - you have to get some kind of external power supply before you can use your HDDs with OTG.