Okay I will respond to these questions numbering them in the order that they are asked. (just not make this easier to follow)
1. Squads, yes just like any other card you have to research and purchase them. You currently have 2 squads (everybody does by defualt) the Gunners SF and Scouts M.
2. I would state it is better to play everything as you will be able to grab all cards then, and there are many cards which many decks of other nations benefit from, examples of these would be Logistics 8A, Telegraphers K (more aggressive decks like Jute like these cards) and Police. I personally can not see how one can focus on one nation but many do. This is mostly dependent on what and how you want to play. Doesn't really matter as long as you are having fun.... However if you are asking in a competitive stand point (not that this game is very competitive right now) I would say play all as that is generally expected to be competitive in any game (that you play and understand everything)
3. It has to do with lack of players.... All there is to say. Advice is to have decks of differing power and cycle threw them.... like if you don't find a match in about a minute or so (or however long you decide) move to another one and queue up in that. It at least makes wait times seem less horrible and at times actually finds you a match quickly.
4. By bad deck do you mean low power.... I am assuming low power.... If the question is down sides to making a bad deck though then it is playing a bad deck
But there really isn't any actual downside to playing low power and clubbing to pad your stats.... Other than most of the XP you gain on it (after unlocking everything on it) will be useless unless you want to spend gold to unlock it. And the other downside is that you probably really are not improving as a player by doing this, since you are not attempting to play all the decks and definitely not playing against all the decks and wouldn't be able to understand how the game runs at different power levels and not gain knowledge of how to play at them. (that last point only applies if that is all you do. Low power is really fun I would recommend playing it sometimes, however always more fun when done as a duel with a friend) So the downsides just really also depend on what you want to get out of the game.... if you only want to play that one deck and perform really well and never want anything else from this game, then go for it ^_^
And no they are not dumb questions. Questions are the greatest thing ^_^ they show what you want to know. Thanks for asking them, I had fun answering them to the best of my ability.
I do hope that these answers did at least help in some way.
Have wonderful days ^_^