In most cases, including your's here, the purpose of the tubes is to keep the chain away from SOMETHING, be it the skin on your legs, your pants, a frame member or the ground. In this case, the tubes will help keep the chain from swaying and flopping so much due to the tube's stiffness compared to the chain. It will also keep all the little chain pin-ends from ripping at the powdercoat on the forks. I think it is a very satisfactory compromise to do this if a multi-idler arrangement isn't to your liking.
Using PEX tubing will add no measureable friction if the tubes are straight and properly flared on the ends. If you buy some of that bulk stuff from Lowe's that is all coiled up, it won't work. They also sell the 5' and 10' lengths that are straight.
To anchor them, I prefer using old spokes. The longer the better. I make a small loop on one end of the spoke to hook under the idler axle bolt for just one tube, or I make a small plate with two little 8-32 bolts to hold the spoke for two tubes if a double idler is used. That little plate goes under the idler bolt head. The drawing below shows that little plate, but has only the power chain tube hooked to it for illustrationof how the spoke is used. The return side tube is run through an Adell clamp like Catrike uses, and there isn't a return idler in this case.
This spoke method allows the chain to find it's own path and alignment with the idler, but keeps it from moving in the dirrection of the chain. The longer the spoke, the better, because if the spoke attaches to the tube close to the idler, the tube can't move as freely to align with the idler. A simple piece of string will work for the power side chain, too, as long as you don't backpedal! A simple bend here and there will make the spoke hold the tube in approximate aligment and the chain will do the rest.
Since we are talking about chainlines and tubes, I would just like to throw a bit of debate in here. Everybody who has seen the chain routing where the return chain runs through a tube that runs over the seat tube thinks that this reduces the friction on the chain by giving it a straight line. WRONG! It actually creates more friction! Why? Because the return chain has a natural curve to it as it hangs under the trike on it's way back to the der cage. The best possible tube arrangement would be to run it through a tube that has such a perfect curve to it that it didn't touch the chain. Not likely, but a tube that has a little bit of curve to it, like most of our's do, is closer to the ideal condition than forcing the chain to run a straight line. In a straight line, every bit of that chain's full weight is resting on the bottom of the tube. In a curved tube, very litte of it's weight is being carried by the tube. Now, you tell me, which one will create the most friction?
A second argument against running the straight tube system is that to run the tube that way, the return chain has to be pushed just a bit farther to the side to get it to clear where it crosses the power side chain TWICE! This also causes a bit more drag in the chainline.
This is also why I like using the spoke method to guide the chain, so that the chain finds it's own path and doesn't rely on the tube to hold it's full weight. Certainly, the weght of the tube on the chain will not create any appreciable friction.
And, as always, you are entitled to my opinion!
