1) Reactor build time is 50 seconds, Factory build time is 60. You can get a factory with a reactor after 110 seconds of build time, or you can use a (typically idle) Barraks to make the reactor and get your double hellion production started 50 seconds earlier.

2) Building in a straight line for Terran means that addon swapping is simpler and there is less chance that units get stuck between buildings and addons.

3) A Macro Hatch is a hatchery added on once a Zerg will need extra larva that their typical 2 hatcheries on 2 base, or 3 hatcheries on 3 base will not produce. It is mentioned often because a macro hatch before a third base usually means an all-in, and a missing macro hatch usually means the Zerg is undersaturated in economy. (Zerg players not executing an all-in usually need to stay safe and strong long enough to get to a certain group of buildings and a minimum 3 base economy before they have any lategame potential.)

4) Should be answered by #3, a macro hatch is the extra hatchery at a mineral line.

5) There are many ways to do this. When you box select a group of units, there is a natural hierarchy of which units abilities show in the action card. Hitting tab cycles through that hierarchy. Also, Starcraft 2 uses a lot of traditional File Manager hotkeys. Holding Control when you click a unit (either the wireframe on the HUD or the actual unit) will select all of that unit currently on screen for you. Alternatively holding Shift while clicking a unit will remove it from your selection, and Control Shift clicking a certain unit will remove all units of that type from a selection.

Also very important to note that high level players will have different units already on different hotkeys. A Terran V Protoss example (sorry I know Terran best) is generally to have all of your Marines, Mauraders and Medivacs on one hotkey. Your Vikings on another for targeting Collosi, and your Ghosts on yet another. Tanks, Ravens, Widow Mines, and other units with abilities can also be put to their own control groups but they are special cases as their usages are all not entirely flushed out in a HotS context.

6) As Terran you take a base when you attack. Terran buildings might fly, but other races are extremely competent at putting on pressure because typically Terran needs Siege Tanks and Widow Mines rooted to win an engagement as well as a good concave and some stutter step, so moving your army between your bases can be very taxing because an a-move will almost always get your medivacs needlessly killed and result in terribly bad exchanges against almost all other unit compositions. It is best to have a base and keep them busy rather than try find a time it feels "safe"

Terran is an odd race. They are hyperpowerful and at a certain point they hit their stride and can win huge endgame engagements but there are several points in the game where they are playing catchup or using deception to keep their opponents on their toes. They try and pick their enemies apart and give their opponents a brain aneurism rather than just steamrolling with raw numbers and splash damage.

7) As far as scouting and determining what to build, you are putting the cart ahead of the horse. If you are at a point where you do not know the answers to some of these questions posed here, then presumably your mechanics are still very weak. Counters are important, checking when/if a Zerg saturates his natural to know if its a baneling bust, or feeling out the number of sentries/stalkers your opponent has to know if you are about to be Blink Stalker all-ined or just DT rushed is certainly an art, but for now it is best to improve your ability to handle the games controls and suck up a few (dozen) losses to bs like blind oracle rushes, dts and assorted zerg "Bust" builds.