You can get the trial at www.lotro.com/trial.
When you launch the installer it has instructions on setting up an account and key.
As for restrictions a trial player cannot send a tell to anyone who has not added them to their friends list (so be sure to add your friends or they'll feel very lonely). They also cannot use the auction house and a few other restrictions to help prevent gold spammers from abusing the system.
Trial players can be invited to Kinships.
Since the new trial has been started does it include MoM content, the rune keeper and warden?
Yes the trial includes all MoM content. Trial players will be able to play both the Rune-keeper and Warden classes.
Can lapsed account holders use the trial to play for a few days on their existing account?
No. You would have to start a new account with the trial.