Well whether you are fading or not, Intel is your best friend. Knowing what is happening and what has been happening the whole game is key I find in judging what to do in a particular situation. Everyone has their own way of fading and may find doing different things more important.

I find that a fades ability to move around quickly is its best ability. Hence I ALWAYS try to move, no stopping by to heal, heal on the go (try to master mid blink meta). Basically I fly around the map left to right and try to keep the marines busy everywhere.

To address some of your questions. Lets say rines are attacking your hive. Your decision to go back should be based on a couple things. 1. How many marines 2. How late in the game, tech lvl (HAs, jps, full tech etc) 3. Try to evaluate in your head, would my being there really turn the tide (after all a hive can go down in seconds) 4. How well is the hive defended (ocs other higher life forms) 5. how many hives (1 hive you definitely want to be there otherwise possible a loss etc..). These are basically the criteria I try to keep in mind as I go around the map. If you think your team can hold out then sure hitting the base and costing rines some tech might be a good idea.

Part of this thought process and decision making should be based on how you feel you perform as a fade. Sometimes it is crucial to keep a fade around so you may want to think twice about entering certain situations. Some people think, meh I have 50 res if I die ill fade again, but if you keep the res and overflow, you will benefit your team more.

"What the biggest group of light armour marines you'll take on? "
This depends on 1. Opponents 2. Tech level 3. Hives/upgrades. If its lights im comfortable with up to 3 marines, above that have to be careful. Not to say I won't enter a room with more then 3, but I will stay more mobile in a packed room.

"LA's with shotguns?"
Some people may just want to avoid shotguns, especially if you aren't very comfortable fading. If you think you can take on the shotguns you must stay mobile, blink swipe, blink swipe repeat. A walking fade is a dead fade vs shotguns.

"HA's? Do I even bother? Keep them busy? Pray for a quick death?"
Ha's blink and swipe, make them sweat, create a diversion for smaller units to get hits in, try to approach trains in open areas not hallways where you may get stuck. Late in the game the fade is a good diversion unit, everyone like to shoot the fade, which is when the little skulks can nibble at their feet.

"Is adren worth it, or should I stick with celerity?"
If you don't have the hang of blinking I would recommend adrenaline, In fact, I would recommend adrenaline either way. More energy = more blinking = faster movement, you can move pretty much as fast as with celerity. Having said that with 2 hives and metabolize celerity can be viable, although I still prefer adrenaline.

"What about focus? Does it benefit late game for those powerhouse blink+swipe attacks? OR stick with SOF"
I pretty much always go with SOF, As far as I’m concerned the only class that really benefits from focus is a skulk and possibly lerks. SOF will help you track down rines wandering trying to build phase gates, which brings me back to one of my initial statements to constantly be patrolling all around the map, use your ability to move quickly.

This is my take on fading, whether you agree or not who knows, but it generally works for me. There is no best way to fade. I have watched many great fades, and they have differences in their styles of play. One thing all good faders have in common is their ability to take full advantage of blink, stay moving.