when is it too late in the year to repoint with lime putty? Has anyone seen their mortar fail due to weather?

May be when it was too hot!

I would go with the hessian - depends on the weather though, may not be necessary at the moment. Too sunny or windy seems to be more of a problem than drizzle or mist. Heavy rain is a problem for a day or two.
We damped down a lot in hot weather to try and slow down the drying.

Posted By: spoonandforkit if it's too damp / humid it won't go off (this is for NHL though, not putty)

I would have thought it was the other way round - NHL will go off in the bag in a damp/humid environment whereas putty will not go off kept in the tub under a layer of water.
In theory you could keep pointing done with lime putty damp long enough for it never to go off.
I mix up lime mortar with putty and keep it in sealed tubs for ages, I think it will keep indefinitely like this. I have used it after a year or more.