Hi again weedhound and thanks for the questions, there good ones!

To answer the first, the reason for simmering bud for two hours (as opposed to say 1 hour for hash), is purely to allow enough time for the heat to break down the cellular structure of the granular (powdered) plant material, thereby releasing all (or most) of the THC into the oil. With hash, due to the production method, most of the resin heads are pre-crushed before you begin heating it.

Regarding the issue of temperature, that's a tricky one, and is a little bit more subjective, but here goes.
I initially started making my own cannabis capsules after reading 'The New Prescription - Marijuana As Medicine' by Martin Martinez, but wasn't entirely happy with the results or the method described, particularly the temperatures he advocated.

Although the 'boiling' point of THC is around the 200 degree centigrade mark (392 fahrenheit), most experts cite 140 degrees centigrade (285 fahrenheit) as the point that THC begins to evaporate/vaporise, so going beyond this point, even for short periods 'can' affect the potency.
I say 'can' because in some respects the oil acts as a barrier to prevent THC loss by trapping it (some will escape though).

To get even more technical (sorry), here's a guide to what happens to your bud when cooking and at at what temperatures:

a) Between 46 and 50 degrees centigrade (114 to 122 fahrenheit) the inactive THC acid starts to decarboxylate into active THC. (a good thing).
b) At between 50 and 75 degrees centigrade (122 to 167 degrees fahrenheit) the CBD starts to melt (also a good thing).
c) Once you go over 75 degrees centigrade the resin heads start to melt (essential).

The beauty of using a crock pot for this type of cooking is:
firstly, you don't have to watch over it all the time, and secondly, it's a stable, regulated temperature that ensures you get consistent results.

i.e. on a 'low' setting your 'simmering' your bud at around 80/85 degrees centigrade (176 to 185 fahrenheit), which means you'll have converted the inactive compounds into active, melted the resin heads and allowed the THC to bond (at a molecular level) with the saturated fat. But, not 'burned off' any of the THC. Thereby retaining maximum potency.

So, assuming you don't exceed 140 degrees Centigrade, you're fine to cook your bud longer. If it starts to smell strongly however, keep an eye on it.

Lastly, there are some medical and scientific studies that compare the different effects of temperature on the kind of high you'll experience (dependent on the percentage ratios of the CBD, CBN and THC that's left in your oil after cooking), but I'm still working on that element. But as a rule of thumb (for now at least), the lower temperature ranges give more of a head high, the higher temps, more of a sedated, couch lock effect.

God, I hope this makes sense. lol, if not, keep asking.
Cheers