Hi and welcome to the forum. Good luck with your search for a nice T206 Mathewson, no matter which pose you decide.
It's not a big deal that you refer to the cards by number, but just an "FYI" that most collectors simply refer to the cards by the player name and pose. So in this case you would typically hear folks calling the cards "Mathewson portrait", "Mathewson dark cap", and "Mathewson white cap". Some really astute collectors (
) substitute the word Mathewson with "Matty" for short, myself included.
To answer a few of your questions:
1) I've noticed a lot of lower grade cards with perfectly rounded corners as if someone trimmed them to create a more symmetrical silhouette. TPGs don't single these out and downgrade them to Authentic, instead assigning a numerical grade. Why is that?
Most likely what you are seeing is not trimming which caused the perfectly rounded corners, but natural wear over time. Since the wear is natural from being handled over the years, TPGs assign a grade because they are not altered. Some collectors shy away from rounded corners, while others like them, often saying it shows the card was "well loved".
2) Is there such thing as investment grade in your opinion?
Yes, but overall the term is really a marketing phrase, in my opinion. In general, for a card as old as T206 investment grade might be considered 6 or 7 and above. For postwar cards like mainstream Topps, PSA 8 and above would be considered investment grade. Personally, any card you buy is an investment and should be treated as such. PSA 2s can appreciate, too, but as with any investment, the more you invest, the more you can make (or lose).
3) How do I look out for cards that have hidden paper loss or filled-in pinholes?
This shouldn't be much of a concern if you are buying graded in the 4 to 5 range (although some paper loss has gotten by the graders in the past, it's rare to see it). Some things you can do to avoid paper loss would be to A) ask the seller if it has paper loss, B) try to view a good scan of the card before buying , and C) when you receive a card, check it out under good light or use a flashlight.
Finally, regarding soaking, most people soak T206s when there is something stuck to the reverse or the card is stuck in an album and they need to get the card out without back damage (provided the glue is water soluble).
Many collectors do not have a problem with soaking a card for these purposes. The topic has been discussed quite a bit in the past here, and you will learn a lot more by browsing these topics:
Link to "soaking" search