I just measured my mast topper and it is 1 ft out the front and two ft out the back, I have never had anything tangle up on the sail battens, but then again I run my stay line thru a 3/8 water pipe to prevent the stay line rope from getting tangled on anything. If your not planning on using the PVC tubing, longer might be better. I have my jib mounted about 3 inches in front of the mainsail (very close to the mast).
I'm sure a miter saw is fine, I usually use a hack saw to cut the stuff.
The position of the jib on the topper is controlled by the slot (that's the distance between the back of the jib and the main mast). It works better I found and you get the most performance gain if you keep that slot space fairly close to the main (within 3-5 inches, overlap is also fine as in a genoa) and try to make the slot as even as possible. The jib organizes and accelerates the air going across the far side of the main making the mainsail more efficient. Actually I think the main purpose of the jib is to direct accelerated air across the main preventing the airflow over the main from breaking down in questionable conditions (like upwind). Basically the mainsail is always the engine, the jib is like an efficient fuel pump or turbo charger.
Basically with a jib you can always point much closer to the wind, and in lighter wind than would normally work with just the main (so I'm told). You can typically sail a little faster with the jib. However when the wind increases too much because you have more sail area, you typically need to furl sooner, that's why on a TI a furling jib is much more handy, as wind increases you just furl it smaller. At least on my TI, if the wind gets extreme, I typically put the main away completely and just sail using the jib alone.
One thing you need to keep in mind on the TI is the main mast is mounted so far forward, there is hardly room for a jib. It really helps if the jib is tilted and mounted further forward at the base, leaving the top close to the main (even if this means the back edge isn't parallel with the mast), what this does is helps create lift and prevents the bow from diving under water.
If you ever decide to add a spinnaker, you will want that at the very tip of the bow sprit, with the jib set back some, and the spinnaker at the front of your mast topper with some separation between the jib and spinnaker (so you can run both at the same time).
If it were me I would run the PVC the full length of the mast topper and cap the ends with something like this.
I have these type caps and spinning fittings on the end of my masts so the masts can be easily furled. After mounting the hardware into the cap, I just glue it onto the end of the PVC tubing. They would also work nicely on the front and back of your mast topper.
Here is another pic of one of those spinner caps (on top of my jib)
You can always put a pipe clamp around the mast topper tubing and put a ring on it (top of sail would clip to ring), this way you can tweak the position back or forth on the mast topper to tweak the position that works best for you.
Hope this helps