Upgrades: Upgrades can be applied any number of times, however, they overwrite the previour upgrade of the same type. For example: an inscribable scythe could have any number of inscriptions applied to it, but only the last one apllied would work, the previous would be lost. Note that a scythe has three possible types of upgrades: an inscription, a prefix (like Zealous scythe) and a suffix (like "scythe of fortitude"), combining into a "Zealous scythe of Fortitude with an inscription.
Battle commendations: You cannot sell battle commendations to merchants. You can only trade them with quartermasters for items and then sell these items. Usually, it doesn't pay to sell - just trade them for identification and salvage kits, and you'll save a lot of money that way.
Skill sequence: Some skills have activation times, some don't. Those skills without activation times can be activated at any time, even while activating another skill. Those that do have an activation time (including all attack skills - they activate as fast as you can attack again) are executed in order. For example, a dervish clicks three skills: Faithful intervention (activation time 2), Chilling victory (attack skill), and Conviction (stance, no activation time. The character will start by casting for 2 seconds, then it will begin to swing as fast as the scythe allows. Conviction, however, will be activated instantly when the button is pressed, even during the casting of the first skill (even though it was pressed third).
Salvage kits: The difference between salvage kits is what you can salvage from an item. With a normal salvage kit, you can only salvage crafting materials. With an expert kit, you can also try to salvage a magic component, if the item has one. For example, you have the Zealous scythe of Fortitude from the above example. If you use a normal salvage kit, it will be destroyed and yield some salvage material. If you use an expert's kit, you will be given an option to salvage either the inscription, or the prefix or the suffix. You will always succeed, but usually, the item will be destroyed. If it isn't destroyed, you can attempt to salvage the remaining upgrades.
Economically, I would suggest the following
guideline: Identify all blue items and above, Identify all items, even white, once the enemies that drop them go above lvl 17, salvage only upgrades you can use or that are readily marketable, salvage for meterials only if you see a highly salvageable item.
Stacking: Stacking means putting one thing on top of another. If you see a bonus that mentions "doesn't stack", this means that if other bonuses that also have "doesn't stack" are present, only one bonus is applied, the highest one. For example, if you were to place three minor vigor runes and a major vigor into your armor, you would only get +41 health total, not +131, effectively wasting all but one rune. Other bonuses that don't have the "doesn't stack" will still work (like the bonus life from the weapon, insignia, ...). Also note thet different bonuses will work. For example, one armor with a superior vigor rune, a superior "Attribute X" rune and a superior "Atribute Y" rune will yield all three bonuses. Also note that all penalties will always successfully stack...
Attribute requirements: You can use any item. However, if you do not meet the prerequisites, it will not function fully, but will have the same function as a starter weapon (minimum damage). Upgrades will still work, IIRC. Your scythe would then only work at 4-6 damage, despite the higher potential.
Costumizaton: You cannot use items customized for other players. Not ever. An NPC can customize a weapon, but CAN'T UN-customize. Customization simply means the weapon is adapted to you. Forever. The only one that can use an item that is customized apart from the player that customized it is a hero of the same character. DON'T LET ANYONE SCAM YOU AND SELL YOU ANYTHING CUSTOMIZED!
The strange guy: Sometimes, GW hosts "party" events, like halloween. For these events, some special fun items are created to enhance the fun. The player in the screenshot is using two of these: a halloween tonic that transforms you into that creature for a while and a snowman summoner that simply summons a snowman for a few seconds. All these items are single use only and cannot be used in PvP.
The specific questions: if you have +30 life from insignia and a single vigor rune, you will get all the bonuses. There is only one "doesn't stack" bonus, and it will thus work. Just don't add more vigor runes, as they won't work. More insignia, however, would work, as would the little +10 health runes.
Accounts: you have one "account", to which you can add more chapter keys. You get +2 character slots for each, and ALL your characters can then play ALL of the content. Your dervish will be able to go to Tyria and Cantha, etc. Do note that you can already play with other people that use that content (see all those white and oriental guys in Kamadan? Well, they can't be from Nightfall, can they

You will only be able to CREATE a new dervish as a primary class in Nightfall, though. Oh yeah, some content is only available to characters created in that campaigns, like the tutorials and duplicate quests.
All your characters will be able to use the same Xunlai storage account as soon as they can open one. This does also mean that the total storage you can have is that one account.
PlayNC - don't ask me.