Mysql Schema design & Rollback necessity Question

[Lightingale]

Hi there,
I am new to using mysql. I want to prepare an application for my employer. The 
application will be accessed by staff from as many as 10 different departments 
such as sales, marketing, admin, finance etc. The users will be using DML 
commands on the tables. My question has two parts:

Part I:
While designing the schema of the database, I have two choices:

Scenarios:
1. Create multiple tables, one for each department. The relationship for most 
of the tables is one-to-one.
2. Create one master table so that each department updates its respective 
columns in the same table. 

Please advise which choice is better. 

Questions:
1. With single table will table locking become an issue if multiple users edit 
the table simultaneously or is it something that mysql can handle without 
problem?
2. What is the maximum recommended size of a table for mysql? How many columns 
should be master table should have ? Is it recommended to design a master table 
having more than 200 columns?

PART II:
Secondly, I am using PHP, Mysql, ADODB, APACHE on windows 7 platform. This is 
my typical DML command:

      $query="update users set 
id='$id',password=\"$password\",pin=\"$pin\",hint=\"$hint\",fname=\"$fname\",lname=\"$lname\",manager=\"$manager\",deptt=\"$deptt\"
 where username=\"$myuser\"";
      if ($debug && $dbgusr == $ses_username) { echo("$query"); }
      if (!($rs1 = $db->execute("$query")))
      {
         DisplayErrMsg(sprintf("Data Select Error: %d:%s\n", mysql_errno(), 
mysql_error()));
         return 0;
      }
      else 
      {
//         updatelog($id,"users","$query","usrmgr.php",$ses_username,$myip);
         DispMsg("User Profile edited successfully");
      }

I am not using any rollback statement to rollback the db if the DML command is 
not completed successfully. Is it advisable to use rollback? If it is how 
should I modify the above statement to include it ?

Thanks in advance for your help. 

Regards,
Lightingales