Just put normal test pads next to the tag connect, a bit more spaced out. A bed of nails in the production line connects to that, the tag connect can be used for bench development.
A lot of low population density cellular backhaul runs over satellite. Two factor SMS or voice can be directly intercepted, assuming encryption is not used.
A solar panel is like a normal silicon diode. The voltage over the cell goes up drastically with lower temperatures. In cold climates crowbars are usually added.
Usually yes, but it is not an EU-wide system. Contact your spectrum regulator and ask for what options are available for private 5G[1]. You can likely get a bit of spectrum in the 1.8GHz guard band, around 2.3GHz, or somewhere between 3.4-5GHz. Price is from free to thousands of euros, depending on the country, transmit power and whether you are using outdoor cells. A private mmWave cell will almost everywhere need to be under an experimental/R&D license (we don't have public mmWave networks).
If you can choose, it's best to take a TDD license, as the duplexer required for FDD base station operation is a relatively expensive device when operating on an uncommon band.
1) Private 4G wasn't really a big thing in EU, but in most countries the license you get is technology neutral and can be used with a 4G base station as well.
PS: Connecting the private LTE to the public telephone network with a publicly reachable number requires a public telecom operator permit in some countries. You will also need to pay for the block of phone numbers. Roaming to the public cellular network requires many extra things which are not cheap or easy to obtain. And of course you need to have a roaming agreement with the other operator.