Harvesting yeast from a commercial bottle can be a little tricky, and requires close attention to sanitation.
If you don't want to bother with harevesting yeast, some people have suggested using Belgian Strong yeasts to replicate Ommegang beers (White Labs 570 or Wyeast 1388)
There are also reports that Ommegang's yeast is White Labs 410
If you want a good Saison, I would personally use a Saison yeast. The Ommegang yeast is not a traditional Saison yeast, which is probably why they use the various spices to give it character.
I think your grain bill looks good, except I am not sure about your amounts. When I put the recipe in BrewToad I was getting a much higher OG! I would lower the dry malt extract from 2 lb to 1 lb in order to get a original gravity around 1.067. You are correct that Hennepin is made with Pilsner and Pale malts, but I would probably switch out the Golden Light LME with a Pilsen LME, and use Light DME instead of the Pilsen DME. No oats reportedly in the real thing, but they won't hurt. Randy Thiel (former Ommegang brewmaster) said that they use simple corn sugar instead of the expensive belgian candi sugars, so you may wish to skip the candi sugar if it is hard to obtain and simply use corn sugar. You will want to up your sugar percent to 15-20% of the fermentables. Currently you are about 13-15%, however, if you reduce your Malt Extract in order to get a lower Original Gravity, the sugar percentage will increase slightly. What efficiency are you planning on getting for the partial mash portion? 70% efficiency?
Do you have capability to do a full boil (8 gallons)? Or are you constrained to a 3 gallon boil? If you are doing a partial boil, you want to keep the gravity of your wort the same as a full volume boil in order to get the proper hop utilization, and then add the Malt Extract at the end of the boil. There is a section in "Brewing Classic Styles" about how to calculate extract additions for a partial boil.
Hennepin has a original gravity of 1.069 (16.8 Plato) and a final gravity of 1.010 (2.6 Plato), which comes out to be a pretty high 85% attenuation. I doubt you will be able to get an attenuation of 85% when using Malt Extract. Accordingly, I would focus on reaching the final gravity of 1.010 more than attempting to get the high 7.7% abv. You may wish to consider lowering your partial mash infusion temp from 150 to 148 (for 75 minutes) to try and squeeze out some more fermentability.
The 25 IBU target for the hop additions is right on; however, the full ounce of Saaz might be a bit strong for the style. Perhaps reduce to 0.5 oz?
For 5 gallon batch, try 4 g Coriander, 4 g SWEET orange peel (Sweet, not bitter according to Ommegang website), 1 g Grains of Paradise, and 0.25 tsp of dried ginger. Add at 10 minutes in a hop sack and pull out at flame out.
Check out what I've thrown together for you: