How much coffee would be appropriate to add to a 5 gallon porter partial mash recipe? Also, should it be placed in the mash, or added to the boil?
When I make a coffee porter I make a coffee concentrate with 1/4 pound coffee and 1 quart of cold water. I put the coffee & water mixture into the fridge for 3 or 4 days then filter it through some cheese cloth. I add the coffee concentrate at bottling time. By making a cold coffee concentrate you prevent any bitterness from the coffee from affecting the beer flavor. If I were to choose between the mash or the boil… I would choose the last minute or two of the boil in a grain bag.
Yeah, cold brewed works best. Avoids the bitterness and acidity like chezteth says. Add it at bottling time to get the most bang out of it.
Neither. Add coffee to taste at bottling or kegging time. You can also “dry bean” in secondary for more aroma.
Regarding the addition of dry beans to the secondary, that sounds appealing but shouldn’t I be concerned with contamination of the beer at that point?
No.
The beans are roasted, so assuming they haven’t been rolled in mold before you throw them in the fermenter there should be very little in the way of critters on them. And for the same reason we don’t worry when we dry hop in the secondary, there’s no need to worry with beans - the low pH and alcohol do a number on a lot of bacteria/mold, as do the hops themselves and the lack of food for many bugs.
RDWHAHB 
What? Drunk and can’t sleep??
Pop some light roasted beans in a hot oven on a cookie sheet until they crack some more- then cool. This’ll sterilize them and wake up the flavor before steeping.
Might smoke a little… 
As Tom said, no worries about infection. If you do this, I usually use 4-6 oz. of coarsely cracked beans in a sanitized bag. I usually leave them in for 4 days to a week.
Neither. Add coffee to taste at bottling or kegging time. You can also “dry bean” in secondary for more aroma.
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I also usualy do the dry bean in secondary. I also have made it by dumping shots of espresso into the keg to taste. both ways can make a great beer.
Before you do ‘dry beaning’, try it on a small scale and see if you like it.
I don’t like the flavor that way; I think coffee (like spices) needs some heat (not necessarily boiling) to develop the best flavor.
But tastes are subjective. Try alternatives and use the one that tastes the best to you.
I’m agree with Gordon on finding your own path to coffee beer enlightenment. I can honestly recommend against “greenbeaning” unless you enjoy putrid beer. 