Clash-of-clans – What does the red symbol with a shield and two swords next to clan members’ names mean

This symbol means that those player can participate in clan's war, The other players who don't have it are spectators and they can't participate in war.

The logic behind that is like this :

• If a player join your clan after you started the war they will become a spectator and will not be able to actively participate in the clan war.
• If your clan members are more than enemy's clan, The players who have lowest trophies will become spectator so both clans have the same number of members ( e.g 30 vs 30 or 45 vs 45)

Yes, you are correct. The clan flag symbol listed with the troops that they used means that they deployed troops given to they by another player in their clan. Check this link out for more info on the clan castle and it's functions: http://clashofclans.wikia.com/wiki/Clan_Castle.

There is no way to communicate with other players except through the global channel, your clan chat, and clan mail (so no private messaging).

While it might seem unfair that someone can use troops given to them by their clan, a large portion of the game is based around players being a part of a clan (it's even in the name - Clash of 'Clans'). Joining a clan can provide many benefits, and sooner or later, you should - and probably will - join one.