28 days plus letter writing time is fine. You can start trays of things now for planting out as soon as she's gone - some stuff like courgettes and tomatoes and runner beans are better not planted outdoors until mid to late May anyway in case of late frosts, salads will grow quickly, purple sprouting broccoli for next spring - and other brassicas, and leeks can go in at that stage too. Salad things generally take 6-8 weeks.
I think once she's got the letter, wandering by every couple of days to take notes (literally, have a notebook, write in it) on things like aspect and sun path and what is already growing where so that you can rotate crops would be a good idea. Chat to other plot owners, ask advice (you may be offered plants, take them) stress how much you've been looking forward to getting started, how disappointed you are about the delay, but how you felt you should be kind to Barbara.
Stress how noble and put-upon you are. Without actually criticising anyone outright.