Is there a way to become proficient in using a Spiked Chain as a non-improvised weapon?

The spiked chain was an exotic reach weapon in D&D 3.5, but is absent from D&D 5e. The use of such a weapon (probably) falls under using improvised weapons.

Is there such a way for a character to become proficient with a exotic/improvised weapon without pursuing proficiency with all improvised weapons?

How many attacks does an Adaptive Shifter get when using the ranged ability from Spiked Form?

The Adaptive Shifter Archetype can select a reactive form called Spiked Form, which when taken twice, grants the Shifter the ability to shoot their spines as a ranged thrown natural weapon multiple times.

…She can learn this form a second time, allowing her to fling her spines as thrown natural weapons that deal piercing damage, have a range increment of 30 feet, and allow her to make multiple attacks with her spines as part of a full attack; these otherwise deal damage and overcome damage reduction as her shifter claws.

The thing I’m confused on is how many attacks the shifter gets, because as I understand it, the number of natural attacks you can make in a full attack is limited by how many you have. Normally, I would think that this means that you could only attack once with the spines, but the text specifically calls out making multiple attacks with the spines.

The two suggestions I’ve seen (from only one person, there doesn’t seem to be a lot on this) are either only getting 2 attacks from the spines (since normally you’d only have two claw attacks, on which the damage is based, except that Adaptive Shifter can change their two claws into one bite, gore, or tail slap) or that it’s based on BAB iterative attacks (except that normally natural attacks don’t interact with BAB). Like, currently it seems like RAW is as many attacks as you have spines, which is ridiculous as it calls out spines all over your body in the first half of the description. Any thoughts…?

Can a Spiked Gauntlet worn by a polearm-wielding Fighter allow him to do attacks of opportunity with it?

If a fighter is wearing Spiked Gauntlets wielding a Ranseur, does he threaten both the 5′ area around him with the piked gauntlets and the 10′ with the Ranseur which would allow him to make an attack of opportunity with either weapon depending on the distance of his opponent?