To answer your questions:
1) Read the wikis on all finishers (projectile, whirl, leap and blast) as well as combo fields. Like other finishers their effect depends on the field they’re used in. While Engineer has access to many fields and finishers comboing them solo is less of a chore or necessity than it may seem. Placing and Finishing combos is not he only thing an Engineer is good for. And just about all professions and specs will have some combo utility that a group can utilize whether it be fields or finishers.
2) Power Rifle with either Bomb Kit or Flamethrower is probably best for leveling with “Strong” (Power > Precision) gear.. Don’t waste too much time trying to min/max while leveling as you’ll be replacing gear fast enough. Once you get access to 3 stat gear you’ll be looking for Berserker (Power > Precision = Ferocity). Attack speed is not something you have any control over from gear stats, and minimal control over regarding traits (some of which aren’t the best choice in their slot). Furthermore proc effects generally have a cooldown meaning they will only proc every so many seconds. When playing Power Rifle you shouldn’t worry at all about damaging condition effects because you’re putting zero resources into condition damage. And investing in condition duration (for non-damaging conditions) likely won’t give you the same return as putting those resources towards damage.
3) Mortar Kit is great utility with good damage on Toolbelt (traited) as long as your target is rather stationary. You get Poison, Ice, Light(Blind), and Water Fields as well as a Blast Finisher and Mortar #5 also counts as an Elixir.
4) Unless you’re playing in a more competitive game mode like raids, league PvP or organized guild WvW play what you like just with other Professions. You’ll find out for yourself what works and what doesn’t, and make changes accordingly. You obviously want to get the most out of your character so you’re not going to do something ridiculous like have a completely willy-nilly spec and gear with mis-matching runes, etc.. Engineers have a lot of tools at their disposal. Pick the ones you like that are effective regarding what you want to achieve and I’m sure you’ll do fine. And as always gear appropriate to the task. The only Turrets that are generally used are Healing for the extra condition removal vs other heals, and (traited) Mortar (see build below). Rifle is also used in Static Discharge builds, but that’s about it. Current Power builds typically run as few as one kit depending on which build you’re looking at, and the build below only utilizes the Toolbelt skills of 2 of the 3 kits slotted. So unless you’re planning on the insane-o Grenadier Raid/Fractal builds I wouldn’t let kit swapping discourage you since it would be no worse that weapon swapping on any other profession. Mine has limited use as a Boon-strip/Blast finisher, and Rocket Boots are typically only taken as a movement ability.
5) Don’t try to compare this to WoW. You are likely overthinking things and driving yourself crazy for no reason. You won’t know how any given profession will play at 80 until you’re there, or test out build in PvP, which I would recommend. Don’t let the feel of a profession in PvE at level 18 deter you.
If you wanna camp a single weapon/kit I would say go with Ging’s suggestion.
https://www.youtube.com/watch?v=3i0aDQKg438
Try different things and see what DOES fit your play style.
