6.1 is the latest for Diamond.
HTC will not give 6.5 to the Diamond, the only way to have 6.5 on your Diamond is to flash a custom ROM.
Read 2 posts above your post to get custom ROMs and learn how to flash your Diamond.
And NO, you can't update or flash with wifi.

You want it to vibrate + ring when receive call or vibrate only? You can change the setting here :

Start  -> Setting -> Sounds & Notifications -> Notifications

Select the event as  - Phone : Incoming Call

Select the Ring type you wanted. E.g. If you want the phone to Vibrate and Ring at the same time, select : Vibrate and Ring.
In regards to the battery life, yes, its a known issue since the beginning. Some of us heavy users have 1 day battery life as well. If you wanted a longer battery life, either :

- Get an extended battery which is thicker but give extra hours

- Flash to other firmwares. Some say those cooked ROMs offers better battery life as well.

- Flash to WinMo6.5 which some of the people here have tested and gives exceptional battery life.

Again, those flashing of firmware can void your warranty and possibility of bricking your phone. So, do it with care

htc touch got accelerator meh? can use gsensor?

Soft is like u restart ur computer, but hard is like format ur pc, u loose everything. U need to do soft reset every 3-4 days to clean up its rom for it to run smoother. For hard reset, usually u only do it when there is some serious prob wif ur phone which cant solve by soft reset.

then perhaps you should take a look at Beyzacase for HTC Diamond which i've been using it before i shift to HTC HD even HD now im using beyzacases too. i dont like those flip top flip side case (maybe im only) i prefer EZ access to my phone.

http://forum.lowyat.net/index.php?showtopic=629892&hl=

i order from this forumer, he quote me the price around rm120!

maybe u guys should take a look at it