Issue in item level permissions for document library with check-in check-out enabled

i want to implement the item leve permissions for a document library whose versioning is enabled and check-in and check-out functionality is also enabled.

i am tryng with the below

1) Everyone should get the ContributeNoDelete permission

2)  Author should get the contribute permissions

3) a custom group "npd admin" should get the contribute permission

whenever i upload a document to this doc lib, its asking me to check-in the document and it remains as checked-out.

when i forcefully check-in through code, the modified by is updated as system account,!! what  shoudl i do to avoid showing it as "system account" and forcefully check-in the doc.

am trying with the code below:  but the issue am facing is , after forcibly check-in code am  geting the modified  by as system account

                    SPSecurity.RunWithElevatedPrivileges(delegate()
                    {

                        SPSite CurrentSite = new SPSite(properties.SiteId);
                        SPWeb CurrentWeb = CurrentSite.OpenWeb(properties.RelativeWebUrl);
                        SPList CurrentList = CurrentWeb.Lists[properties.ListId];
                        SPListItem CurrentListItem;
                        SPWeb myRootWeb = CurrentSite.RootWeb;
                        CurrentListItem = CurrentWeb.Lists[properties.ListId].GetItemById(properties.ListItem.ID);

                        SPFile spfile = CurrentListItem.Web.GetFile(CurrentListItem.Url);
                        SPFolder spfileFolder = spfile.ParentFolder;

                        if (CurrentListItem != null)
                        {
                            CurrentWeb.AllowUnsafeUpdates = true;

                            if (!CurrentListItem.HasUniqueRoleAssignments)
                            {
                                CurrentListItem.BreakRoleInheritance(false);
                                CurrentWeb.AllowUnsafeUpdates = true;
                            }
          
                            
SPRoleAssignmentCollectionSPRoleAssColn = CurrentListItem.RoleAssignments;
                            
for(inti = SPRoleAssColn.Count - 1; i >= 0; i--)
                            {
                                
SPRoleAssignmentroleAssignmentSingle = SPRoleAssColn[i];
                                System.
Typet = roleAssignmentSingle.Member.GetType();
                                
if(t.Name == "SPGroup"|| t.Name == "SPUser")
                                    SPRoleAssColn.Remove(i);
                            }
                        }
SPUserspevery1nodelete = CurrentWeb.EnsureUser("Everyone");
                            
SPRoleDefinitionmroleDefinitionevery1nodelete = CurrentWeb.RoleDefinitions["ContributeNoDelete"];
                            
SPRoleAssignmentspRoleAssignmentevery1nodelete = newSPRoleAssignment(spevery1nodelete);
                            spRoleAssignmentevery1nodelete.RoleDefinitionBindings.Add(mroleDefinitionevery1nodelete);
                            CurrentListItem.RoleAssignments.Add(spRoleAssignmentevery1nodelete);
                            
SPUserspUsrEnsuredUser = CurrentWeb.EnsureUser(currUser.LoginName);
                            
SPRoleAssignmentmroleAssignment = newSPRoleAssignment(spUsrEnsuredUser);
                            
SPRoleDefinitionmroleDefinition = CurrentWeb.RoleDefinitions["Contribute"];
                            mroleAssignment.RoleDefinitionBindings.Add(mroleDefinition);
                            CurrentListItem.RoleAssignments.Add(mroleAssignment);
                            
//this.EventFiringEnabled = false;

                          
//  if (CurrentListItem.File.CheckOutType.ToString() == "None")

                            {
                               CurrentListItem.File.CheckOut();
                               CurrentListItem.SystemUpdate();
                                
//CurrentListItem.Update();