Extending top-rope masterpoint over edge - safety while setting up a long extension?

I've been reading John Long/Bob Gaines "Climbing Anchors" and have been practicing these techniques both at home and at the crag. Yesterday I came across a top-rope anchor building scenario that I couldn't find a good answer for in the book (or online) - ultimately, I knew how to rig the anchor, but was unsure how to safely get the anchor to where I needed it.

Scenrio: A vertical wall of about 70 feet that transitions to a low angle slab at the top (~20-30 degrees). About 15 feet up the slab there are two bolts for rigging an anchor. The rock on the slab is quite rough and abrasive and the edge where the slab transitions to the vertial wall is sharp enough that the extention needs to be protected from rubbing. Access to the bolts is gained by a very easy and safe walk-up around the backside of the crag. The two bolts are the only viable anchor points available in the 15 feet between the bolts and the edge transition.

I understand the rigging for extending the masterpoint over the edge (in this case I would be using static rope for the extention setup - configured similar to the example shown below, but with carabiner & bolts vs. slings & natural anchors):

 
Where I got stuck was when I needed to walk down the slab to the edge to setup and position the masterpoint, pad the edge and clip-in and drop the top rope - what should that sequence of events look like, where and how should I be tethered and clipped in while setting up the edge?
Best idea I could come up with in my head would be to leave a long tail on the static rope, setup on the static tail for a rappel to the edge (ATC with third-hand) with a knot in the tail well before the end of the tail and before the edge of the cliff, rappel to the edge, lock off, setup the masterpoint and top-rope, walk back up to the bolts (removing slack through the rappel device as I ascend), unclip and walk back down the trail to the base.

I'm sure I'm missing a lot of important factors and considerations here - for that reason I didn't feel comfortable improvising, so I didn't do this - I just went to a different section of wall with easier and more straightforward access for an anchor.

Hoping the more experienced here will provide feedback as I would like to use this as a learning opportunity so that next time I'm at the crag I can anchor this section safely (the climbing on the vertical section of wall is perfect for my current skill level and never very crowded).

Thanks!

You'd be psyched checking out the SPI Manual...tons of efficient, useful techniques for protecting oneself while working out an edge/top out/exposed anchor....

https://www.amazon.com/Rock-Climbing-Single-Pitch-Manual/dp/0762790040

What you described is fine imo. I would get yourself a Gri-Gri or similar device for work at height as it makes scenarios like this about as foolproof/oops-proof as it gets. The Gri-Gri allows you to either jumar or climb and pull slack your way back to the top if you aren't rapping down and if you are rapping down, it's easy and safe to transition from your Gri-Gri on a fixed line to your ATC on your top rope.

It just makes these type of scenarios so much safer and easier to work with. Obviously any of the other similar devices work just fine, but i use a Gri-Gri so that's the example i went with.

If this slab is low-angle enough that you can basically walk up and down it, but you just want a backup of some kind, you could fix the static line to the bolts (or just one end, so you can decide the length for your masterpoint when you reach the edge), then just put a prusik (or two) on the static line, and clip it to your belay loop.  This will require a bit of tending to make sure it stays cinched enough to catch while still sliding along the rope, but it's very quick and easy (and cheap, too!)  When you reach the edge, just pull the prusik tight, and sit on the edge to set up your masterpoint.  This should feel very secure.  If the slab requires real rappelling/ascending, I wouldn't recommend you try it this way.  A grigri will do basically the same thing, but better, so I second that suggestion if you have one handy or plan to do this a lot.  In case this isn't obvious, this is a 100% static set up with the prusik and your static line, so don't allow slack to build up, and don't use this kind of system if there's any chance you'll take a significant fall on it.  

Lots of good advice here, but I'll add my 2 cents. Not every climb is suitable for top roping; and not every climb that is suitable for top roping makes sense to top rope without leading up to the anchors.

Like others, assisted braking belay device backed up with a friction hitch knot (Gri Gri + Prusik). Lower to edge to set then jumar back to safety. Use a long tail off the static or another rope attached at a secure anchor point. 

Prussik on your static line is enough for backup when walking back on that easy slab. When you get the extension and master point all set up you can then use your ATC to set up a rappel on your TR and just rap down to the ground. 

I would just add to make sure you put a stopper knot in your lowering line.

Is it kosher to use the tail of the clove as the safety line?

Ah, great, man! I did an SPI and exam before finishing off the full guiding courses/exams at the AMGA. The SPI stuff shouldn't be discounted at all---I learned a ton doing the program! Glad it worked out...tons of good techniques and tools in there! RC