Starcraft – Is it considered a loss if you logout or close the game during the countdown

You have many options here. Many protoss players use proxy pylons placed in strategic locations on the map so you don't have to divert your attention too far away from the battle in order to warp in new units. You can even go so far as to bring a probe with you in battle to make pylons a short distance behind your army to help warp in units and also provide supply for the large army you're creating.

Hot keys can be used to great effect here if you practice a bit. Double-tapping a hotkey will center the screen on that location, so if you have a pylon or other unit near a pylon where you want to warp hotkeyed, you can center location on it with a hotkey, hit W (to select all your warpgates), shift+click a bunch, and then double-tap a hot key to go back to your army in a second or so with practice.

Remember also that it's generally a good idea not to queue units, so warp gates enforcing this is a good way to practice your macro for other protoss production buildings and if you ever plan on playing Terran. You generally want to be spending your minerals on things you get now, rather than things you will get later. This is an important point which is mentioned often by Day9 in his tutorial casts.

Most of all, just keep practicing.

Yes.

You should receive full points. No part of the point calculation or the change to your rating is reflected by anything that happens in game.

On an unrelated note it is considered proper etiquette to say "gg" before leaving in such a manner.

Now I'm sure someone will mention pro-matches in reference to this question. When participating in a tournament with Bo5 style matches (Best of 5 games, or First to 3 wins) experienced tournament players will actually delay a final push when they believe they have 100% chance of victory. The purpose is to cause the losing player to exert additionally mental stress in trying to win an unwinnable game and thus have them "tired" for the next game in the series.

While this behavior is not considered bad manners and is often times a signifier that you are playing against a tournament pro, it leads many professional gamers to bow out earlier than you might normally expect. Because they know they still have to play potentially 4 more games, top players will often quit if their opponent has gained a significant advantage at any point in the game.

The corollary to this behavior is that in a game N situation (or game N-1 if you're losing floor(N/2) to floor(N/2)-1), losing players will often stay longer as defeat implies elimination.