Usually, the RSVP are sent around 2-3 months before the wedding, but you can also send some Save the Dates earlier to make sure your guests "save the date" Will you send your invitations by mail or e-mail?
Yes you need to make the website on the desktop version. But after they're is a mobile version to use
Save the dates are usually sent out 6-8 months before the wedding... if it is a destination wedding then 8-10 months! For invitations they are usually sent out 6-8 weeks before the wedding, and again for a destination wedding you will want to give them more notice so closer to 3 months! You can work on the wedding website on your phone but I find you can't see as much of the information... for example on my homepage I can't see how many days until the wedding where is on the desktop you can see the full picture !