TP 3-1/8 in. x 7 in. 20-Gauge Galvanized Tie Plate

Yup. It is sheet metal with holes in it. If you have a bench vice, you can make straight bends.

Use a HTP which is thicker gauge (16 ga.) and made to tension loads up to a certain point. Check the Simpson Strong-Tie specs for exact figures. It will do the job whereas the TP series is more of a non-structural mending plate type of connector. Using the proper type & number of fasteners is important too.

You can, but a HTP would be better. Also, unless you have engineering/architectural design qualifications, and especially where a govt. agency inspection is required, I would strongly not recommend doing so. JMHO. :)

You'll only have a little over 1.5" of "bite" on each board if you orient them like you propose. The boards will be joined, but won't have a very strong connection. The wood is liable to split screwing that close to the edge. I suggest you put two plates side by side on each side of your 2" x 8" boards. The plates aren't that expensive. Also, if you use 2.5" screws, they'll poke through the other side of the 2" x 8" board, since a board of that dimension is actually only 1.5" thick. No problem using a #8 screw, but a 1.25" length would be better. I used the Simpson Strong-Tie plates to join 2" x 4" pressure treated shed rafters where the roof pitch changed for an overhang. Philips head screws pulled in and made a "dimple" in the plate to make them almost flush. As for your application on the ground into pressure treated lumber, the plates are galvanized, and will take years to rust. You could paint them if you want them to last longer. Use marine epoxy and they'll last even longer.

If the rim joist bears no weight and is non-structural, this plate should work to simply mend in a new piece

Yes, for non structural applications

8d nails or #9 SST 1-1/2" screws

Nope. You would have to use the HTP series connectors which are a thicker gauge and have limited load-tensioning ability.

Nope, these are not meant for a load

Yes