Well, this depends on the type of ticket you purchase. As far as I know (not 100% sure as I usually buy Sparpreis tickets when travelling on my BC25), you'll only receive a discount for the person holding the BC when purchasing full-fare tickets, but you'll receive a discount for both when purchasing a discounted ("Sparpreis") ticket (as long as you purchase one ticket for both travellers). You cannot buy a ticket for someone else using your BC discount.

Unlike FFPs, the person who buys the ticket will get all the credits. This also applies when purchasing tickets for two or more people where not all of them hold a BC. Per 1 EUR ticket price you'll receive 1 award point and 1 status point. It's impossible to split credits between two bahn.bonus accounts when travelling on a single ticket.

Yes. But not the other way round.

Those "trial upgrades" are random promos which have been quite frequent in recent years, but I haven't had an invitation for some time. However, you should know that the "trial upgrade" will NOT upgrade your tickets to first class, but only the BC discount. So, if you happen to sneak such an upgrade voucher, your BC 2nd class will also grant discount on 1st class tickets, but you'll still have to buy a 1st class ticket.

They don't require a DB segment, but yes, in order to profit from international discount ("RailPlus"), your journey needs to start in Germany. Not sure whether third-country cross-border segments (e.g. Austria-Switzerland) will also be eligible for RailPlus discount from a DB BahnCard.

This only applies to the bahn.bonus card which is NOT a BahnCard (so no discounts) but allows non-BC holders to participate in the bahn.bonus loyalty scheme.

Correct. You'll receive elite status as soon as you have spent 2'000 EUR spent in a rolling one-year period. You'll receive a silver BahnCard for replacement, and as long as you'll keep this spending limit, all subsequently issued BahnCards will be status cards.

Not at all. While I don't know about the restricted fares (I'd suspect they'll only be available when there's a Sparpreis available), full-fare awards (such as the 1st class free ride for 3'000 points) don't really have to be "redeemed" in a way FFPs require it - you'll simply receive a voucher which you'll present to the conductor instead of a ticket. There's no such thing as "award space" on DB, award and upgrade vouchers will work on any train, at any time.

Hm, the lounges aren't really noteworthy.. in the usual bahn.comfort lounges (found at many major train stations), you'll just receive free soft drinks and coffee (self-service), in the dedicated 1st class lounges (Berlin, Hamburg, Cologne, Frankfurt Central, Munich) you'll additinally receive small snacks and alcoholic drinks (beer, wine) served by waiters. Nothing to write home about, and nothing compared to even the most basic airline lounge.