In response to your first point, no one's ability is not governed by their position, but odds are a player will play a position best suited to their ratings. You're not going to have someone with P Manning's ability playing DT, or someone with Ngata's ability playing FS.
How much does STR determine a successful block? I guess I've always been a little unclear what the STR rating does, if its really that important in determining a success block. I always assumed that the blocking rating is much more important in determining a success block than the STR, but I could be wrong. Would Welker indeed get run over if he lined up as a TE because he has a low STR value? My main point is that if Gronk and Welker were lined up on the line and asked to block a LB, Gronk should have a much higher success rate.
Looking at Welker's ratings, I'd say he fits the mold of a TE better than a WR. He's not fast, just quick. Great receiver in the short passing game and is a good blocker for a TE. His only downside is his STR rating, which again I'm not sure how much this rating matters. If he'd make such a good TE, why hasn't NE switched him to TE yet, let him and Hernandez trade positions?