I do not share the experience that anal sex is rarely messy or the opinion that an enema is unneccessary for anal play... Maybe someone who eats tons of greens or has a diet very high in raw fibers would not have any material hanging out in their rectum between bowel movements, but with the average meat and starch based diets out there, trust me, most of us need an enema for anything beyond a couple fingers or a simple prostate massage. It's a big disappointment when a guy spends a lot of time relaxing and working himself down onto a dildo and then immediately has to stop playing because his own fecal matter is causing him internal abrasions. I don't tell guys to clean themselves out because I'm afraid of poop... I am not really bothered by it beyond the inconvenience of having to clean it out of the towels afterward. The problem is that it is extremely irritating to your own insides if you are being penetrated by any object that feels large to you (which is different for everyone) and there is fecal matter rubbing and causing unpleasant friction on your insides. When I get to the point in play where I start to see fecal matter, it's the guy who undoubtedly tells me he needs to stop within a few minutes of that. Practicing good anal hygiene is important for anyone who enjoys anal play as its own "main event"and will be disappointed if they have to end that type of play early.
As for the original questions...
1) use whatever kind of water you drink to clean your rectum. Your body is used to it. If you drink filtered water, put filtered water in your ass. Add a tiny pinch of salt to mimic the salinity of your body's own fluids. By using the water your insides are familiar with already and mimicking your body's own salinity, you create a minimally disruptive experience for your body.
2) in the same vein as described above, the least disruptive temperature for your enema is your own body temperature. Obviously, no thermometer necessary... Just approximate!
3) there are a lot of great douche systems out there if you really want to get into it. The best ones, designed for experienced players, can be found most commonly in gay sex stores or online. One of my friends has a shower unit so he can adjust the temperature and then just fill himself with gentle water pressure. I usually just use fleet enema bottles filled with my own water, but I just ordered a new douche called the "Booty Call Booty Blaster" which is made of body-safe silicone... I'll be sure to let you know if it's awesome! The name sure is cool. I would say to shop around and see what appeals to you and experiment until you find something you like.
4) yeah... That just happens. Most people I know try to do their enema like three hours beforehand so that stuff works its way out of your system well before playtime begins. I try to do a variety of sitting and standing to help my body shift around and let any remaining liquid work its way out of me.
5) I don't know because I always use a squeezable enema. I would imagine the ideal speed of flow is a personal preference and you just need to experience. A higher bag will empty into you faster than a lower-hung one.
6) the size of the enema that is best depends on how deep of play you are engaging in. Prostate massage vs dildo vs giant dildo vs fisting all involve different depths and increased likelihood of encountering fecal matter as you get deeper into your body. I don't think two quarts is necessary unless you're being penetrated with large objects. But I suspect you might be doing just that!
7) I love to be on my knees with my face in the ground when I give them to myself, but this is again a matter of personal preference in my opinion. Maybe there is someone who knows even more about enemas who can tell us if any one position is better than others. I haven't tried them all or noticed any major differences for laying on my back vs on my knees. One thing I have noticed is that being more comfortable helps me relax which makes everything go smoother, so I would suggest picking something that's not too uncomfortable.
8) I'm also a fan of hands and knees next to the toilet. Really any floor will do though!