According to the press release, Doria will be dropped off at the castle. That's where Meghan will pick up her pageboys and bridesmaids, before continuing on to the church. The release says members of the royal family will start making their way to the church at this time, so it's safe to say Doria will be in the church before the Queen arrives.
I remember the designer was on hand during W&K's wedding. You can see her fixing Kate's train right before she walked down the aisle, so I imagine the designer of Meghan's dress will be the one helping her with any last minute fixes.