My second was 8 lbs 7 oz and I had an almost 4th degree tear as well (he was also four weeks early!). He had his arm up so I delivered his head with his elbow!
My third was even bigger, 8 lbs 11.5 oz, but his head and shoulders were smaller and there was no arm to contend with, so I only ripped a little (my midwife said I could have one stich or not, so I chose not) and I did just fine.
Since my midwife delivered my 2 and 3rd and knew after my second that I had such a horrible tear, I'm a little miffed that she didn't tell me my chances, or even the existance of, the complications you mentioned!!
I don't know if knowing that would have changed me having a vaginal birth or not, so I'm kind of glad I didn't know, but all the same, everything turned out better the last time around.
As for post partum infections, the chances are higher for C-sections, but I'm sure you know that already. Also, think about the mandatory down time with a C-section versus the possibility of down time if nature takes it's course. It's a little like roulette. With a vaginal birth, the amount of down time is a game of chance. With a C section, downtime is almost always a must (and you'll be trying to keep up with your first). I know lots of women who do it, though.
The placenta thing? I just don't know. That is one thing I never had to deal with.
This probably didn't help you a whole lot! I'm sorry!
All I can tell you with any certainty is that every pregnancy, labor and delivery experience is different! This next one could be completely opposite than before, even with your previous issues. Good Luck!!!