the dps calculation in the spreadsheet are all done for autoattacks only right?
Is there a benefit of using slow weapons because they scale better with skills?
Yes DPS in my spreadsheet is strictly from autoattacks. There is a column showing damage per hit too. But, DPS with ability use will depend on which abilities you use and in what order, I cannot cover all those possibilities. But, autoattack damage is an important part of your overall damage and so I wanted to be able to calculate it.
There is a benefit to using slower weapons, which generally have bigger damage per hit, with you skills, yes. Look at the tooltips for your skills; those already have your weapon damage calculated in (including offhand ability contribution, OHAC). So you can equip different weapons and see how much baseline damage you will do with them. I am pretty sure that does not include your stats like +1h/2h/ranged dmg, and definitely not positional (because it depends on your positioning!). But, those are big factors too.
In the end the only way to really know what does the most damage overall is to go to a target dummy and do a timed rotation of attacks on it, then (manually) tally up the damage. (Tip: If you are on a Mac, you can screenshot the combat log and use the Preview app's built-in OCR to copy the text out. It's pretty reliable for numbers. Then you can use a text editor to copy out the damage numbers. If you are familiar with grep-style find/replace, an app like BBEdit makes that easy.)
Dual wield is great for Wardens because they have a number of effects that can proc each hit. It's also great for any situation where you want lots of attacks per second (mob shields with # hits and such), or for Berserkers when they are out of stamina (there is not universal agreement on that however). Another area where big damage/hit is better is in AOE attack situations, so Berserkers and Juggernauts generally want to use big slow weapons in those situations.
I like dual wield because, although you might not get those giant blows like with slow weapans, that also means you are less likely to pull aggro, because you are doing a thousand cuts rather than a single big smash. A smoother damage curve over time also means your foe might get a hit or two fewer on you because you knock its health down steadily rather than in stair-steppy chunks. It requires a good pair of weapons however, which can take some farming or crafting to obtain, whereas with a two-hander, you just have to find one weapon.