Not sure how much yeast nutrient you should add, but I'm guessing that there are some recommendations on the container somewhere. As for the yeast, you should be good with a starter. If you have some time before you're planning to brew you could also step the starter up to 1L.
I've used yeast...
I've found that it's best to always give your honest opinion. Sometimes new brewers don't respond well to negative feedback, but in the end it will make them a better brewer and they will probably thank you for it - especially if you can offer them suggestions about how to correct the faults...
I'm not sure. I could have made some adjustments on the fly, or had some settings (SRM, etc) incorrect in the version of Beersmith that I was running at the time. I If you want to bump them up, I would recommend increasing the base grain slightly.
As for the addition I made:
I've made this...
I typically dump the liquid, leaving just a bit on top of the yeast, swirl it all around to get it mixed up and make sure that I'm not leaving clumps of yeast behind, then dump that in when it's time.
I also know some people that don't decant their starters, ever... and they make absolutely...
The best answer is 'it depends'. If the yeast cake/trub is below your spigot, then you can just transfer right from there. Sanitize the spigot, some tubing and the bucket/carboy/better bottle that you're transferring into and then open it up. The risk of damaging your beer by doing this is...
I'm not a big pumpkin beer fan, but I've tried a number of them. 2011 ST Pumking was much better than the 2010 version, but I'd still have to say that my top pumpkin beers are Weyerbacher Imperial Pumpkin and New Holland Ichabod.
On average I'd say 3 or 4 weeks. If I do a session beer that I can just drink the heck out of while I'm working on projects, then that may only last me 2 weeks.