And don't stir them in. Just drop them in. Or if you want to make sure they don't float on the top, bag them with sanitized SS washers/weights and then drop them in.
For 5 gallon batch, secondary is not necessary. Weight of the beer is not enough to cause any problems. Even with dry hop you'll only be on the yeast 2-3 weeks, so no issue.
depends on the yeast on what off flavors you might get.
You don't have to repitch. As you said the lager yeast was going like crazy. 90 degrees wont kill them off, they'll just throw off flavors.
You'll still want to boil for 5-10 just to sanitize. Since the extract is prehopped it will provide some bitterness, but maybe a 1/2oz of magnum in there for the 5-10 boil would be a bit of a help.