1.) The official procedure for exposed support cloth is to fill it with glue, let it cure, then patch over it.
2.) Edge seam leaks are very difficult to patch. You can sometimes do so with thin fabric, then burnish it well to form a perfect "90 degree" angle against the seam. Pennel & Flipo ORCA 215 is a fairly thin and strength csm/neo fabric you could utilize.
3.) They're overpressure valves.
4.) For best durability, you should touch the transom up with a 2-part polyurethane type paint, with a flattener added. I used Interlux Brightsides w/ flattener additive on mine with good success. The paint is vital to protect the wood from rot.
For glue, you'll need a 2-part elastomer type chloroprene glue for use with inflatable boats. Bostik 2402 w/ Bostikure "D" is one of the popular canned variants available that you should be able to find.
Gluing geek since 2007