In or out of the saddle is kind of tricky. The idea of FTP is to measure how hard you could theoretically go for an hour. The limiting factor on riding for an hour is your body's ability to clear the lactic acid being built up in your legs. Over the course of an hour, you will be much more efficient in the saddle, and you will clear lactic acid more quickly while seated as well. For a 20 min interval, you can probably spend some time out of the saddle and might increase your output, but it will not be a effort that is truly sustainable for an hour, as your legs will fatigue and burn out before you reach that point if you are doing too much out of the saddle. Generally, you should try to do most of it in the saddle, but if standing a bit from time to time allows you to maintain the same or greater effort while using slightly different muscles, that is a reasonable thing to do.
Cadence is somewhat personal, and people often feel like they prefer slower cadences, but when you actually start working at it, riding a faster cadence like 85-100 is almost always more efficient than riding slower than that on longer efforts if you train for it. You might be able to do 70 cadence with more power for 10-15 mins, but at minute 30 or 40, fatigue in your legs will be the limiting factor, whereas the same effort at 90 cadence would be sustainable longer.