Namkeen is a salted and/or spiced savory (as opposed to a mithai, which is a sweet). Namkeen are snacks, usually served with tea (not appetizers, they are not served with a meal).
Kachori is a type of small, filled pastry that is then fried - it may be served as a namkeen or as a part of a thali (for a meal). It usually has a savory filling but they may have
dried fruit (i.e. like the Vyas-brand ones which are also usually quite spicy, or other sweet fillings) in them. i.e. all namkeen are snacks, but not all kachori are namkeen.
Vyas brand kachoris are produced in India & are, I believe, not exported to the US, so no, you're unlikely to find them here as your friends say. If anyone has them it will either be Patel Bros or Kamdar Plaza, but I think they all carry the Haldiram brand, which is exported to the US (Patel also offers non-branded namkeen in their bulk barrels, they may have kachori). I'm not aware of any of the sweets shops on Devon who offer locally-made dried fruit-filled kachoris (Rajbhog used to, but that shop has closed; Sukhadia only offers savory khasta kachoris; Kamdar also offers probably the best variety of locally made namkeen, they might carry the fruit ones).
ETA - sazerac, what you have written is correct, but its not the type of kachori that Vyas makes. Its a different beast entirely, a speciality of Jamnagar. (And there is a huge variety of other sweet kachori available).
ETA2: ohfreddie, if you call & they don't have the Vyas brand, ask for Jamnagar kachoris to get the same type.