OK, I'll give an opinion since everyone seems to be mulling this one over.
I'm not going by your number system because my answers don't necessarily answer YOUR questions - sorry
, will kind of all at once!
1850 house - probably didn't have 1x face nailed plank flooring, they were much nicer by then. Face nailing pine plank in 1850 would be a CHEAP(read poor) way of doing the floor. So while you are looking for old-timey-charm, this probably isn't HC. That being said I don't see why you couldn't put these floors in your new kitchen addition and say this is the look you were trying to achieve.
Also, in response to one of your questions(2), someone is making square nails again, pricy but available. Also, try to find someone tearing down a property - they will have plenty available just waiting for your "help". Square nails are easily straightened and reused - them suckers are durable!
Veneer flooring and carpeting - hmmm. You suspect the original floor is under the veneer? I suspect the original floor is under the carpet! Find out what you have first, original flooring should get you your old house feel good. Refinish what you have and remember, it is probably in much better shape than you think. Just cleaning off the decades or dirt can do wonders to some floors. You need to enlist Vaso, I believe that's who it is, they are the current floor champions with their latest 104 yo douglas fir porch floor clean up! Hey Vaso, what did you put on those floors anyway - you said you'd be embarrassed to say and nobody called you out on it! Give it up!
Answer to #4 - "Take my kids..... please". OK, we only have a 15 year old so someone else will need to donate a couple as well! The ultimate stress machine is a 3 yo!