Just swim up to the cut box and practice using your tools; a few lbs in the bottom of it should keep it down. One other thing to add to the mix is an old LP or HP hose. It's important to know how to cut things, but also important to know, by feel, what not to cut. Please do not intentionally entangle yourself solo; that kind of drill should only be done in a pool with experienced support divers.
A z-cutter or z-knife is a hook shaped cutting tool with a protected blade originally designed to cut parachute lines and stolen by divers for cutting lines. They're particularly good for reaching behind you and cutting lines hung up on your first stage. Get one with a small enough mouth that none of your hoses will fit in it; should still be more than big enough for any line you run into and minimize the bubbles produced when cutting blindly behind yourself. Incidentally, they also work great for cutting seat-belts.
As far as dangers... low viz is dangerous, solo is dangerous, diving itself is dangerous. The trick is to minimize the risks and know what level you're comfortable dealing with. There are a lot of knowledgeable divers here, and everyone manages risks a little differently. Most control risk by training, equipment, experience, and planning. Many solo divers solo in areas well known to them and dive very conservative profiles, some carry ponies or doubles, and some divers don't solo at all.
A good way to think of it is task loading. Everyone has some level at which they get overwhelmed. Good training means that buoyancy, fin technique, gas management, .etc are almost unconscious. That means that whenever you get a new situation, you don't have to concentrate on maintaining the fundamentals while you deal with it. Current, low viz, entanglements, disorientation, overheads.... all of these add tasks to your load. Can you manage those which you expect to see on your dive and have enough left over to deal with an emergency? If not, I'd think very hard about doing that dive.
Training helps minimize the task loading; it's generally pretty easy to clear an entanglement if you stop and address it slowly, deliberately, and immediately. Thinking through scenarios like you're doing is also a great idea; it helps streamline the subsurface head scratching. Gear helps to an extent as well; a streamlined rig is easier to swim and less likely to snag, a redundant air supply means you can shut down and fix a regulator problem without worrying about losing your gas.
The biggest key, to quote a song, is to know when to hold 'em and know when to fold 'em. We've all had days when we get to the water and decide that it's just not my day to dive. Don't dive plans that you're uneasy with, and don't be afraid to turn a dive or not dive at all if it doesn't feel right. That's one of the greatest things about diving solo; it's a piece of cake to change your plans and there's no buddy to get fidgety if you just hover watching the sun and shadows on the quarry wall for half an hour, to pick a purely theoretical example.
Sorry about the epistle length post, hopefully it helps some.
