OK, you asked for opinions, so I will give you mine. Keep in mind what they say about opinions though.
Question 1 Navy has handled items from several makers over the last 50+ years, but I am under the impression that currently, most of their revolvers are Pietta made.
2 I have no experience with their Frontiersman, but I have two Pietta 1851s, and am satisfied my money was well spent.
3 The fit and finish is decent enough, considering the price. If you spend a little more for revolvers from Cimarron or Taylors, they seem to be fit and finished to a little higher standard.
PIETTA
I have a Sheriffs model 1851, but it is an old Hawes. I also have a 5 inch Uberti 1858. They are both easier to carry and get out of the leather, but in my case, I find target acquisition faster, and hit more consistently with the longer barrels. A word of caution. Avoid the brass framed ones, as the brass frames are not real durable. I use a 5 inch long piece of 1/2 inch PVC pipe to aid ramming with those short loading levers.
UBERTI Pocket Navy.
I don't have a Pocket Navy, But I do have it's counterpart, an 1862 Pocket Police. Basically the same gun, but with the 1860 Army style round barrel and "creeping" loading lever. Also fluted instead of engraved cylinder. It is fun to shoot, very accurate, but you give up one chamber, and about ten grains of powder per shot. Also, I don't find the small grip frame real comfortable. The fit and finish are nice, and they seem strong enough for the 15 grain max charge they use. They do use the same frame as the 31 caliber 1849.
UBERTI 1849
I have a friend who has two of them (one he converted to cartridge). They are cute little pistols, and he loves them. I have shot his on occasion, and can say they are a lot easier to control and hit with than my 31 caliber 1863 Remington Pocket. A pound of lead lasts a long time when casting bullets !
LONG AND SHORT. Don't buy until you have a chance to handle all of them. After that, it is just a matter of personal preference, and you will be the best judge on that.