I'm in the same position as you, Katy: Half mara PB of 1:34, and I'm targeting 3:30 for my first marathon in April. Something quicker might be theoretically possible but I suspect it could be a grim experience and potentially risky. I'd rather have a bit in the tank for the last few miles and negative split than risk blowing up.
I don't think there's any problem at all in just running through a cemetery. Historically the great Victorian cemeteries were meant to be places to be enjoyed for healthy recreation, after all. There's a couple on my regular routes, and I hardly ever see anyone in them. It's all about sensitivity to other people - I would stay well clear of anyone visiting a grave, and would turn round rather than go anywhere near a funeral. Lap training - why not, if it's empty, but if it isn't empty it would seem distracting and inappropriate to me, and given that I wouldn't want to be nervous about annoying other people, I wouldn't plan a full session there.
Ian - brilliant achievement raising the pace on the last three miles. My mile splits completely fell to pieces on the greenway. Huge commiserations to those looking for times who didn't realise the full marathon had been pulled. The late breaking news on the event website declared the race WILL GO AHEAD bold caps, which I thought was rather a hostage to fortune. But the main thing for this one was surviving - I came home to a local road closed by a huge tree right across it; make no mistake, this was a proper storm - there will be much, much better opportunities for pbs to come!