Full Implementation
1️⃣ IAppLogger
public interface IAppLogger
{
    void Debug(string message, Exception ex = null);
    void Info(string message, Exception ex = null);
    void Warn(string message, Exception ex = null);
    void Error(string message, Exception ex = null);
}
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2️⃣ NullLogger
public sealed class NullLogger : IAppLogger
{
    public void Debug(string message, Exception ex = null) { }
    public void Info(string message, Exception ex = null) { }
    public void Warn(string message, Exception ex = null) { }
    public void Error(string message, Exception ex = null) { }
}
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3️⃣ Log (Facade)
using System;
using System.IO;
using log4net;
using log4net.Appender;
using log4net.Config;
using log4net.Layout;

public static class Log
{
    private static IAppLogger _current = new NullLogger();

    public static IAppLogger Current
    {
        get => _current;
        set => _current = value ?? new NullLogger();
    }

    public static void Debug(string message, Exception ex = null) => Current.Debug(message, ex);
    public static void Info(string message, Exception ex = null) => Current.Info(message, ex);
    public static void Warn(string message, Exception ex = null) => Current.Warn(message, ex);
    public static void Error(string message, Exception ex = null) => Current.Error(message, ex);

    public static void SetDisabled()
    {
        Current = new NullLogger();
    }

    public static void SetAdminFile(string adminLogPath)
    {
        var fullPath = ResolvePath(adminLogPath);
        EnsureDirectoryExists(fullPath);
        ProbeWriteAccess(fullPath);

        var layout = new PatternLayout("%date %-5level %logger - %message%newline%exception");
        layout.ActivateOptions();

        var appender = new RollingFileAppender
        {
            File = fullPath,
            AppendToFile = true,
            RollingStyle = RollingFileAppender.RollingMode.Size,
            MaximumFileSize = "5MB",
            MaxSizeRollBackups = 3,
            StaticLogFileName = true,
            Layout = layout
        };

        appender.ActivateOptions();

        BasicConfigurator.Configure(appender);

        Current = new Log4NetAdapter();
    }

    public static void SetAdminAndUserFailure(string adminLogPath, string userLogPath)
    {
        var fullAdmin = ResolvePath(adminLogPath);
        EnsureDirectoryExists(fullAdmin);
        ProbeWriteAccess(fullAdmin);

        var layout = new PatternLayout("%date %-5level %logger - %message%newline%exception");
        layout.ActivateOptions();

        var fileAppender = new RollingFileAppender
        {
            File = fullAdmin,
            AppendToFile = true,
            RollingStyle = RollingFileAppender.RollingMode.Size,
            MaximumFileSize = "5MB",
            MaxSizeRollBackups = 3,
            StaticLogFileName = true,
            Layout = layout
        };

        fileAppender.ActivateOptions();

        var memoryAppender = new MemoryAppender();
        memoryAppender.ActivateOptions();

        BasicConfigurator.Configure(fileAppender, memoryAppender);

        Current = new Log4NetAdapterWithFailureFlushWithPath(memoryAppender, userLogPath);
    }

    public static void SetUserFailureOnly(string userLogPath)
    {
        Current = new UserLogOnlyAdapter(userLogPath);
    }

    internal static string ResolvePath(string logFilePath)
    {
        return Path.GetFullPath(logFilePath);
    }

    internal static void EnsureDirectoryExists(string fullPath)
    {
        var dir = Path.GetDirectoryName(fullPath);
        if (!Directory.Exists(dir))
            Directory.CreateDirectory(dir);
    }

    internal static void ProbeWriteAccess(string fullPath)
    {
        using (File.Open(fullPath, FileMode.Append, FileAccess.Write)) { }
    }
}
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4️⃣ Log4NetAdapter
using log4net;

public class Log4NetAdapter : IAppLogger
{
    private readonly ILog _logger = LogManager.GetLogger(typeof(Log4NetAdapter));

    public void Debug(string message, Exception ex = null) => _logger.Debug(message, ex);
    public void Info(string message, Exception ex = null) => _logger.Info(message, ex);
    public void Warn(string message, Exception ex = null) => _logger.Warn(message, ex);
    public void Error(string message, Exception ex = null) => _logger.Error(message, ex);
}
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5️⃣ Log4NetAdapterWithFailureFlushWithPath
Fixes included
✔ flush only once
✔ user appender created once
using System;
using log4net;
using log4net.Appender;
using log4net.Core;
using log4net.Layout;

public class Log4NetAdapterWithFailureFlushWithPath : IAppLogger
{
    private readonly ILog _logger = LogManager.GetLogger(typeof(Log4NetAdapterWithFailureFlushWithPath));
    private readonly MemoryAppender _memoryAppender;
    private readonly RollingFileAppender _userAppender;

    private bool _flushed;

    public string UserLogPath { get; }

    public Log4NetAdapterWithFailureFlushWithPath(
        MemoryAppender memoryAppender,
        string userLogPath)
    {
        _memoryAppender = memoryAppender;
        UserLogPath = userLogPath;

        var fullUserPath = Log.ResolvePath(UserLogPath);
        Log.EnsureDirectoryExists(fullUserPath);
        Log.ProbeWriteAccess(fullUserPath);

        var layout = new PatternLayout("%date %-5level %logger - %message%newline%exception");
        layout.ActivateOptions();

        _userAppender = new RollingFileAppender
        {
            File = fullUserPath,
            AppendToFile = true,
            RollingStyle = RollingFileAppender.RollingMode.Size,
            MaximumFileSize = "5MB",
            MaxSizeRollBackups = 1,
            StaticLogFileName = true,
            Layout = layout
        };

        _userAppender.ActivateOptions();
    }

    public void Debug(string message, Exception ex = null) => _logger.Debug(message, ex);
    public void Info(string message, Exception ex = null) => _logger.Info(message, ex);
    public void Warn(string message, Exception ex = null) => _logger.Warn(message, ex);

    public void Error(string message, Exception ex = null)
    {
        _logger.Error(message, ex);

        if (_flushed)
            return;

        foreach (var loggingEvent in _memoryAppender.GetEvents())
            _userAppender.DoAppend(loggingEvent);

        _memoryAppender.Clear();

        _flushed = true;
    }
}
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6️⃣ UserLogOnlyAdapter
using System;
using log4net.Appender;
using log4net.Core;
using log4net.Layout;

public class UserLogOnlyAdapter : IAppLogger
{
    private readonly RollingFileAppender _userAppender;

    public UserLogOnlyAdapter(string userLogPath)
    {
        var fullPath = Log.ResolvePath(userLogPath);
        Log.EnsureDirectoryExists(fullPath);
        Log.ProbeWriteAccess(fullPath);

        var layout = new PatternLayout("%date %-5level %logger - %message%newline%exception");
        layout.ActivateOptions();

        _userAppender = new RollingFileAppender
        {
            File = fullPath,
            AppendToFile = true,
            RollingStyle = RollingFileAppender.RollingMode.Size,
            MaximumFileSize = "5MB",
            MaxSizeRollBackups = 1,
            StaticLogFileName = true,
            Layout = layout
        };

        _userAppender.ActivateOptions();
    }

    public void Debug(string message, Exception ex = null) { }
    public void Info(string message, Exception ex = null) { }
    public void Warn(string message, Exception ex = null) { }

    public void Error(string message, Exception ex = null)
    {
        var loggingEvent = new LoggingEvent(
            typeof(UserLogOnlyAdapter),
            LogManager.GetRepository(),
            "User",
            Level.Error,
            message,
            ex);

        _userAppender.DoAppend(loggingEvent);
    }
}
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