We ask plan sponsor to send us copy of notice when it is complete (we send them one that needs them to fill in a few items before posting/distributing) and keep the copy with their adoption agreement.
As far as low participation, you can set up an administrative policy to get enrollment forms where employees indicate they don't want to participate, but are you going to get them from everyone each year? Maybe just have the employer keep them on file if you feel the need to have them completed.
Obviously you can't force participation, so provided the notice is properly delivered and the employer is following a set procedure for soliciting all employees to participate, I'm not sure there's much else you can do.