Firstly, have you flashed your tablet yet with the latest Ampe A85 r4.0.2463 firmware yet? If so, was it via Livesuit or Phoenix Card? - If you didn't do it via Phoenix Card (which I know works), you might have some of the various bootloaders corrupted from earlier attempts. This could manifest themselves in the manner you describe in that you have problems powering on etc (see below).
Charging your tablet
Yes, you can charge your tablet using the micro USB socket - I do this pretty much all the time. The only drawback I know of is that is could take longer than using the mains charger if the system limits the current/power available (which I've not checked).
Power up process
The time it take for the power up process for me is as follows:-
1. Press power switch, and the green led comes on.
2. 4 seconds after the green led comes on, the Ampe logo appears,
3. 10 seconds after step 2, the screen turns off & back on and the Ampe logo appears again,
4. 21 seconds after step 3, the screen blanks, displays the Ampe logo quickly followed by the 3 i's
5. 27 seconds after step 4, the main Android 'lock screen' appears.
Total of about 62-65 seconds to boot from powered-off state to the main Android 'lock screen'.
IF any of the bootloaders (there's about 4 of them) are corrupted, it's entirely possible that you might not see anything on the screen even though it will eventually boot. As an example, if you boot it from cold with it connected to a TV via HDMI, you'll see nothing on the TV until step 4 above, but you will see all the steps on the LCD.
As far as rooting is concerned using the Tasselhof script, option 3 is unnecessary because we already have the Google Play store & apps installed.
I'm no real Android expert (but I do have over 40 years of computer experience - lol), so I may be wrong about the reasons, but hopefully my explanations make sense in describing why your problems might be occurring and how to solve them. One way or another we'll get you back up and running
Let me know if I can help in any way.