Kind of a blanket 'yes' to all the questions. :):)
Lots of other companies made pickers, but NI & JD made the last best ones. NI had a terrific husker; JD had a terrific header with stripper plates.
NI made mounted pickers that fit on the tractor - a large amount of time to mount & unmount. They also made 1 or 2 or 3 row pull type. Put in the pin, hook up the pto, & go picking.
One row is unseen in my part of the world - would take all day to get anything done. I see 1 row is real popular in other regions.
I'm not so good with model numbers, A6 was an old 2 row can't find much for parts; 300 was a newer better 2 row but not easy to get parts; 324/327 is the 'new' modular design which NI supports quite well yet for parts. (Did they quite production somewhere in the '80s?) Those are the 3 I have owned....
Some of the very old designs elevated out to the side, but most all elevate the ears out the back to a pulled wagon.
I just got a manual from Eby for my newest picker. NI does have many of them also I believe, for the newer models anyhow.
We pulled the 6A & 300 with a 36 hp IHC 300 and worked well with 100 bu wagons. I pulled the 300 & 324 with a 85 hp tractor & 200 bu wagons, probably a bit more than I need but works fine, some days it's nice to have, actually made it work this fall in heavy corn in the drizzle in peat ground, pulled the engine down - but I'm driving at a good clip, more than they suggest. Not sure what a 1 row would need, but the full wagon takes the power & traction...