Caustic is the way to go and it won't hurt your keg if you're using it in the proper dilution.
Most home brewers won't have access to caustic and instead use PBW or similar products.
I'd suggest BLC as the best choice if you can't get caustic.
This will work with a soak over time, but it's best to circulate it if you can.
Considering it's an alkaline product, you shouldn't use co2 to push it as it will decrease it's ph.
A pump or air compressor will work to circulate the product.
I'm not sure how hot you can use the BLC, but the caustic works best between 150° and 160°.
This should then be rinsed out with water and then followed with an acidic rinse solution. Star San works fine.
I clean my beer lines every 2 weeks and then swap out my lines every other month. I just made up an extra set of lines so I can just quickly swap them out. I'll then let them soak for a while before a complete rinse and hang up to dry.
There is new beverage line that will last longer, but they still need to be cleaned regularly.
The only issue I've had with the lines over time is that they'll get a yellow cast to them.
Beer line is CHEAP so, when in doubt, through it out.
I hope this helps.
Cheers,
Brian