Can the 841ND ver 7.2 be flashed with the 7.0 DD-WRT Firmware ?
Yes, it can. I've done this 13 hours ago and it runs stable since then. Notice that in order to make use of the full wireless bandwidth (802.11n with up to 300 MBit/s) you need to set channel bandwidth to 40 MHz and you have to disable encryption or use WPA2-AES - that's the only 802.11n-compliant encryption (see http://www.dd-wrt.com/wiki/index.php/Wireless-N_Configuration). If you set any other encryption DD-WRT will automatically fall back to 802.11g allthough it would show 802.11n being enabled.
I'm running "v24-sp2 (11/21/10) std" and it has been stable so far. The only bug I've been experiencing is the setting of the WiFi-channel when I use 40 MHz channel bandwidth: when I set the "upper" channel to be the control channel, the lowest channel selectable is 6 allthough 5 should be possible, too. Further if I set the "lower" channel to be the control channel, I can only set channel 4 or "Auto". And if I then try to switch back to "upper" channel mode, channel 4 remains the only choice. After a reboot I was able to set channel 6-13 as "upper" channel again. So there's obviously a bug, probably in the GUI.