Lots of questions so let me help you out when I can.
There are lots of lace wig companies out there & to say one company is better than the other could be difficult. But for your first couple of orders you might want to order with a company like RPG as there are a lot of reviews out there about this company. In time you may feel more comfortable ordering from some of the names you pick up with on the board.
Bleached knots are the knots on the lace wigs which will 'effectively' disappear when bleached, so the wigs look more natural. Ask your vendor to do this. If they don't you could try faux bleaching with sally hanson spray (do a search) until you feel comfortable bleaching your own knots.
Got 2 units to switch it up - they should be the same if your trying to pass the unit off as your own. How long they will last will depend on how you treat them.
Check the newbie threads for maintenance or do a post on what type of hair you are thinking of buying as different hair texture/types requires different maintenance .
Place some of the cut off lace under the band and sew gently & you should not tear the lace.
If you are natural I suggest cornrows and a wig cap under your wig.
If you are using got2beglued or some type of non glue method then yes you will have to take off or it will naturally come off and you will reapply every few days.
As you will have access to your natural hair then yes, it is a good transitioning style as you can moisturize you natural hair regularly.