I can only answer the final question... It can, but it's not recommended to do so. Windows 7 has WAT (KB971033) and with this installed there's a chance that MS could deactivate systems using SLIC v2.2. Uninstall WAT and then you shouldn't ever have any problems though.
Why put your WHS 2011 install @risk to test 2012 essentials? Is there some new feature in 2012 essentials that would warrant the risk? You also have an upgrade path with your Slic 2.1 with either WSS 2008 R2 essentials or SBS 2011 essentials question is do you gain the features your looking for.