Does passing the test make you more marketable? Depends on the group, how much echo they do, what their needs are, how involved cardiology is in intraop echo, size of the hospital, academic vs private, etc. I've got a fair bit of echo training, and some groups were very interested. Others didn't care.
Value of testamur vs diplomate will depend entirely on your place of work. You can do whatever you're allowed to. Where I did my residency, most of the staff were testamurs, simply because they didn't jump through the hoop of doing the paperwork to become a diplomate. No one cared. If you know your stuff and you do the right thing, you shouldn't have a problem. With the hypothetical patient above, if you have done a thorough evaluation of the new valve with appropriate clips saved to document your decision, and you make the right call, you've done your job. That's all knowledge, skill, and experience. It doesn't have to do with testamur vs diplomate status. I'm sure there are plenty of testamurs out there who are far better than some of the diplomates.
IMO, the PTEexam is not the best test of your clinical skill at using TEE to guide surgery/therapy. For example, I don't believe there was a single image of a perivalvular leak on my exam, but they did show a cor triatriatum. I do a lot of echo, and I've seen far more perivalvular leaks than cor triatriatums, and it's more important to be able to identify the perivalvular leak and judge whether it needs immediate attention or will resolve with protamine than it is to be able to identify cor triatriatum.
The difficulty of the PTEexam will depend on your echo education and the amount of study you put into it. There aren't too many places where residents routinely take it. If you've got good teachers and a couple months to really study Sidebotham, Perrino, and know the math from "Conquer the PTEexam" you've got a good chance. Bored Stiff TEE is pretty useless other than as an intro to TEE for the first day of a TEE rotation, so I don't think it's worth the money. Do not forget ultrasound physics. Passing the exam without knowing physics would be challenging, but the physics isn't all that hard.
If you're really serious about echo (more than just passing the PTEexam), add Oh's The Echo Manual, and the books by Catherine Otto.