I'm just going to answer based on my opinions and on what I know. Note: I could be wrong with some.
* It seems that there are a number of these businesses; how does one get a competitive advantage over others?
This probably the most relevant part. Pretty much the only way for people to choose you over the others is that you should have something that you do a lot better compared to others, may it be better user interface/user experience, probably a faster web server, better features, original features(that other casinos don't have), decent fees, etc.
* It seems like switching costs for the user are pretty low. Combining this with the above makes me wonder how challenging they are together.
Pretty much, especially for most(if not all) crypto casinos as KYC is mostly not required. Switching is as simple as withdrawing their funds to another casino. So chances are, crypto casinos as definitely very very competitive. If you're planning on competing, make sure you have something that's a deal breaker for you to at least have a chance to attract/steal the users from other casinos.
* Are there network effects with these businesses? In other words, does it matter how many other people are playing at the casino I am playing at? (If I'm playing against the house.)
Most likely(for the Casino's side though). For the end user though, probably not unless you're playing poker or something, whereas other players are needed.
Casinos make money mostly from their cuts and because the odds are in their favor; if we're talking about stuff like dice games whereas the chances of winning are something like 49.5%, they definitely need more volume to have higher chances of profit.
* At what volume does it actually make sense to run one of these? (I'm sure this is a context-dependent answer - would love to hear perspectives!)
Probably depends on how much profit you're expecting/aiming.
* I see a number of scripts available for sale. Does anyone in the business actual buy these, or do people just do their own custom development?
Probably for the smaller casinos. If you want specific features though, you probably might want to start from scratch or at least heavily rewrite some parts of the code as there might be some conflicts/limitations with those scripts that are being sold.