The metal threshold will support itself over that span with no issue. You won't want to leave it without the boards under it for the long term, though, as it will bend under the weight of people stepping on the unsupported metal, but for the short term of the construction project (where it's getting little to no use and you can remember to not step on it) it will be just fine.
Also, I highly doubt you'll need a saw to remove that wood there, just the careful application of a pry bar. Those pieces should be nailed to the wall and there shouldn't be any attachment to the threshold or the ends of the siding.
Also, check with your contractor - what does he expect to find when she gets there to do the actual install?
- Will he butt the granite directly against the foundation?
- Then don't put anything in their way.
- Will she install a joint expansion strip?
- Is this something you could do for them?
- Is there anything else you can do to make their job easier, or do they want you to stay away so they don't have to correct anything that's not to their spec (and a potential cause for disagreement later if something goes wrong).
You will definitely want to replace the trim after the steps are installed. This way you can ensure that you have a nice tight fit under the door and to the top of the step. If you install it before hand, it might be in the way or even get damaged during the installation.
Composite/PVC trim would be an appropriate material to use here when you do replace that trim material. You'll probably need 2 layers to bring it back out from the outside wall of the house to be flush with the bottom of the threshold.
Be sure to caulk the ends of the trim pieces (both of them) where they butt up against the siding to ensure water can't leak behind.
Be sure to apply a thick bead of caulk to the top of the trim where it meets up with the bottom of the threshold.
- To get the boards slipped between the step and threshold while not wiping all the caulk off, cut a bit of a bevel on the bottom of the trim board so it can go in at a slight angle, top first, then be pushed nice & tight against the wall, the step and the threshold.
Attach each board individually to the wall, just be sure to offset the screws to that you don't run the outer board's screws into the ones beneath.
Ask the contractor what kind of sealant they recommend between the bottom of the trim and the top of the step (if any).
- Not putting a sealant here might be preferred as it allows a path for water to escape if it should happen to get behind the trim. Again, ask them.
You can get screws (with a driver bit) specifically for PVC trim. The ones I got included a driver with a depth stop and a set of plugs to cover the screw heads. They're essentially invisible once the plugs are in place. Worth the extra few bucks in my opinion.
- I don't think nails are really recommended for PVC trim - it's too easy to over drive them and the hammer will dent the material if you miss.