Meat – the purpose of red wine vinegar in steak marinades

I can't see any reason for the marinating itself to make any difference. If it's safe to leave the (un-marinated) meat in the same conditions for the same length of time, then it's safe to marinate it for that long.

Five days in the refrigerator is definitely stretching it for chicken - usually no more than a few days is recommended, and that's assuming it was fresh when you bought it (not a day before the sell-by date). Even if it's safe, it's not going to be very good. I've accidentally left raw chicken in the refrigerator and forgotten about it before, and it started to smell "off" after about 4 days. I hate to say it, but I wouldn't use it at this point.

As for the steak, you're probably okay, because you only have surface bacteria, but I would cook it well and right away. Again, most cooks I know will recommend no more than a few days in the fridge.

P.S. I've heard people say that weak acids such as lime juice "preserve" the meat, but never from a reliable source; even if it worked, every marinade is different and it would be nearly impossible to predict the exact amount of time it preserves for. You have no idea how much bacteria existed at the outset, and even if the marinade somehow helped to inhibit growth of new bacteria, the "spoiled-ness" of raw meat doesn't come from the bacteria itself but from the toxins they leave behind. It's not a requirement for the bacteria to multiply in order for the food to spoil, if enough existed in the first place.

Unless somebody has it on good authority that marinating makes any significant difference, I refuse to put any stock in that bit of folk wisdom and recommend that others be equally skeptical.

To answer part 1. I recently watched the Chicken Tikka Masala episode from Blumethals "Further Adventures in Search of Perfection". The recipe calls for marinading the chicken for quite a long time. A transcribed recipe says 10 hours: http://www.dominicsayers.com/documents/ChickenTikkaMasala.pdf.

They also go on to compare the results of a yogurt marinade and a yogurt-less marinade on chicken under an MRI scanner. I don't know if this is valid, but the yogurt apparently had an effect.