4. My requirement is - When the grid is edit mode and I am going to edit the webcombo type column's cell, the webcombo should auto check the previous choices those I made before.

I enclosed one simple sample based on your scenario as attachment. In my attached sample, WebCombo – that is used as WebGrid edit type, will check the previous choices that made by user before. Adding more choices will be added to the existing choices.

In my sample, this scenario only applied on adding new row, not editing existing row. Please check the attached sample and let us know whether this helps or not.

6. I need the checkbox so is there any way to add checkbox in bound mode.

Adding checkbox in multiple selection WebCombo bound mode can be achieved by setting TextBoxMode to ‘ReadOnly’ and enabling AllowBrowseAll as shown in the snippet code below.

<ISWebCombo:WebCombo ID="WebCombo1" runat="server" UseDefaultStyle="True" Width="534px"
    DataSourceID="ISDataSource1" DataTextField="FullName" DataValueField="Email"
    Height="20px" DataMember="Customers">
    <LayoutSettings TextBoxMode="ReadOnly">
    </LayoutSettings>
    <MultipleSelectionSettings Enabled="True" AllowBrowseAll="true" />
</ISWebCombo:WebCombo>