Hello David,
I've just seen your post and I'm concerned that you're having sleepless nights! You should be having sweet dreams about your new boat - you've made a fine choice! I don't have a P21 but I can assure you that handling the boom is easy - it's light and requires only the removal of a pin at the gooseneck to unship it. (If you leave the topping-lift in place, the aft end will remain supported until you're ready to stow it below.)
You've got the instructions for lowering the mast. Just work methodically and it's fine - I raise and lower mine routinely by myself. Try to ensure the jib pole is guyed evenly to avoid the mast erring to one side but, if it does, you can correct it by pulling on the cap shrouds to guide it down straight.
Have fun! Best wishes,
Anthony Russell
P235/002 Sea Wyche
Anthony Russell
235/02 Sea Wyche