I don't think it's too late for changes.
Your suggestion of three sessions is worthy of exploration. I calculate that 2 squads of 50 will take approximately the same time as 1 squad of 100 but as you say the waiting can take place somewhere other than on the course.
I am quite willing to put that together though mindful that any decision will yield moaners.
If those on the afternoon session understand that their finishing time is approximately the same whether we use 2 or 3 sessions. (We would need volunteers to paint after session 2 which would add more time).
With so many shooters interested I fear for the prize giving ceremony and this will be worse if we use 3 sessions.
My main concern about 3 sessions is that I've no idea who is traveling with who so any allocation would be done randomly. I don't then want a plethora of requests to swap between sessions 2 and 3. However, if afternoon shooters are willing to accept either session 2 or 3 then it's worth a try.
Difficulty now is finding shooters' views