be happy to help as much as can but i warn you im rusty....
mdn = mobile directory number
so basically your phone number
so if your number is 88855512334
then username is
88855512334@vzw3g.com
password is : vzw
it is normal for the SIP password to be blank when you go to the NAM params screen so no worries there
if your programming was wrong there you would not have a 1XEV connection on the device
about the 4.1/4.0 thing...
pre 4.1 the 7250s only gave you a 1X connection
after 4.1 was deemed stable they released a copy of it for the 7250 that let it utilize an EV connection
plus os 4.1 is smoother
so i suggest using it
back to why you cant connect
you mentioned that you are not paying $15 for the tethering feature
this is probably the issue
i havent supportd VZW in a while but back when i was i know they made attempts to restrict tethering from working when the feature was not present
not sure how they are pulling it off
but i remembered when it first came in to existance they were using handset crippling
for example the moto e815 would not tether without the feautre unless you punched in ##DIALUP and enabled it, then it worked fine
but if you added the feature and OTA'd it (*22899) it would unlock it's self and work fine
back to the blackberry
i remember when blackberry tethering first started being supported
if you tried to tether without a tethering feature it would disable mobile ip on the device
this would kill the device's connection (data connection refused)
im pretty sure they fixed this with later revisions of the OS
you could still check it out if you want but i think it would be a waste of time
through hypter terminal or Ctrl + T in vzam type :
AT$QCMIP=?
should return a number, 0 = MIP disabled, 1 = preferred (should be here), and 2 is MIP only
and can be set with the same command
if using HT, you need to enable passthrough first
again i dont think this is your resolution
its been a long time since i messed with verizon and tethering
but maybe someone else here could shed some additonal light
but the easiest solution would be to cough up the $15