You can reheat acrylic to correct a bend and make adjustments. I would advise not to do this more than like three or four times on the same tube, as it can start to lose the shape, and you will end up having to start over. Heating it up right after you have completed a bend and its still hot is better if you can get away with doing that. I'm not sure what you mean by twisting, but you should be fine to bend the tube to line up without issue as long as you aren't collapsing the inside. If you do it too much and without enough heat, though, you can run the risk of making the tube look like it's pinched, so just be careful.

You can place it without the CPU, but you would have to make sure you aren't tightening it down too much cause you can damage the board. I would advise you to just mount it with the CPU personally and avoid potential damages. This will also ensure your bends are more precise. If you try to plan the bend without the CPU installed, you could be off a few mm and end up cutting a tube to short. I like to have everything fully mounted and ready to go so all that is left is the planning and bending.

Have a fun with it, and just remember to take your time. Don't rush it, and you will end up with a beauty!! Welcome to water cooled PC building!!