Hello,
I'm Delphi & C# developer for more than 15 years, I have build many API using both, also I great experience with wrapping Delphi code into API, I have done this for large ERP made with Delphi, and now it's has both XML Web services & Restful APIs.
Regarding your questions
1. Windows service is running application without UI, If you need to build the application as service, you should not consider there's any interaction between users and the application, also for showing results (Showmessages & other dialogs), should be emitted, and you have to use Windows Event viewer instead or log files.
2. You can compile both code as 32 & 64 bit with Delphi, but you use right variables and pointer, for example NativeInt, will allocate 4 bits with Delphi 32, but 8 for 64bit, while integer will remain the same.
The most dangerous operations with code are type casting while consider the datatype or pointers will remain the same.
3. I started to build XML web services with Delphi and I have few problems, specially with older Delphi version, I have moved to RemObjects.
I used Datasnap for API and it works fine with less than 100, but it add unnecessary data to the JSon.
For API I would prefer using Delphi MVC framework or Xdata, or even asp.net web API2, which can integrate well with Azure.
I was not able to write all answers, because proposal should not exceed 1500 chars. please let know if you interested with my proposal so I can send the other
BR,