how to check if member is in group

isExist = IsUserMemberOfGroup(currentUser, groupName); 

ISO format of date needed in list creation via soap

Specify date field value in ISO format, for example 2016-03-09T00:00:00Z

custom list form must exist in e folder for the list

this is must . if form are outside they wouldn't work

Lookup across site collection

Out of box not possible
Third party available..

How to exclude items from a crawl based on items properties

custom creator can be created to fulfill the request

Edit in datasheet does not work with webpart filter

create seperate dataview and then datasheet will work

progmatically setting home page of publishing site

 Publishing sites to set the Home Page.
you can set SPWeb.RootFolder.WelcomePage

User deleted and re-added to AD - Profile not found

You need a temp account on which you migrate the profile first then to the id.

how to update users email if there is no interface in UI

Dont go to database. instead use the power shell to do same

Add-PSSnapin Microsoft.SharePoint.PowerShell
$site = Get-SPSite http://example.com
$web = $site.RootWeb
$user = $web.SiteUsers.GetByEmail("usr@example.com")
$user.Email = "user@example.com"
$user.Update()

sharepoint 2010 sesion state

1. Enter the following PowerShell command in the SharePoint 2010 Management Shell window:

Enable-SPSessionStateService –DefaultProvision

1. On each web application for which you want to use session state, edit the web.config file and set the enableSessionState property of the pages element as follows: