[Iska]

1. Например, не вводить, а указывать из существующих.
2.

Код:

@echo off
setlocal enableextensions enabledelayedexpansion

:Again
set /p sVar=Enter environment variable name:
if not defined %sVar% echo Not found environment variable name [%sVar%] & goto :Again

echo %sVar%: [!%sVar%!]

endlocal
exit /b 0

3.

Код:

@echo off
setlocal enableextensions enabledelayedexpansion

:Again
set /p sVar=Enter environment variable name:
1>nul 2>&1 set %sVar% || (echo Not found environment variable name [%sVar%] & goto :Again)

echo %sVar%: [!%sVar%!]

endlocal
exit /b 0