Can you use this to hide previously installed lines?

It would be very difficult to install this over pre-installed lines unless they are not secured to the wall building or other structure. If not attached you could slide it underneath separate the lines and put a screw through the plastic.

Is there an attachment to snake around a corner?

has a lot of attachments to allow for many configurations. however, you could put straight down and let the water drain straight down and use a separate standard type conduit/insulation tubing for your freon and/or electrical line, which is what i think my A/C guy did b/c i hid my compressor unit behind a corner of the building (on a different side of the building from the air handler unit). i bought this product first and it is attractive. i bought a different one amazon sells (Decorative line set made by pioneer) for another unit and that one comes with flexible PVC line cover as part of its kit. the hide-a-line (this product) is better looking though if you do not mind the conduits/insulation tubes going around a corner.

Does this unit need to be installed by a HVAC tech or can I do it...?

If you are handy you can do some of it but you have to have the lines pressure checked and sealed and that takes know how and special tools. Also there is the electrical part. Maybe you can set it up to a point and hire a guy to finish.

Can this be used on indoor lines?

It's almost universal that mini split lines are run on exterior walls. But I've built double thick interior walls myself to hide them, so I know circumstances can be unique. I see no reason that they couldn't be used to hide inside lines. But if those inside lines run along hollow walls, you'll need to use hollow wall anchors to secure them. Using them in an attic with exposed joists or studs would be no problem. But be sure to use the supplied insulation and tubing wrap for the full length of any run.