A problem with a silent ring is that ringing is needed to trigger the display of caller ID. For the SPx Service you would like to be silent, try setting X_RingProfile to A and X_DefaultRing to 10. This is a ring with 1/4 second on and 9 3/4 seconds off. With luck, you'll still see caller ID, but the phone will make so little sound that the kids won't notice, or can be told to ignore it. You can further tune the ring by editing Ring Profile A -> Ring Pattern 10 -> RingPattern.
To have incoming calls on the Line port not be processed by the OBi at all, set LINE Port -> InboundCallRoute to {}
For your fax dilemma, I have two suggestions. The first is to give up on HelloFax -- you can easily port your number to Anveo, Callcentric, Vitelity or many others, and have reliable incoming fax service at lower cost. There are many ways to sign a document on your own computer, ranging from free software such as IrfanView (simply paste your signature image into the fax image) to high-end packages such as Adobe Acrobat. If you get lots of forms by fax to fill out and sign, the latter may be a worthwile investment, because it is pretty good at identifying the location of form fields -- you just click each box and type.
If you are really wedded to HelloFax, IMO you should spend $10 on an experiment. Get a temporary local number (in your rate center) from a quality provider such as Anveo or Callcentric. Forward it to your HelloFax number and test. If it works reliably, you can be quite confident that when you port your landline, it will also work well. If it doesn't perform to your expectations, you can at least evaluate their incoming fax function, to see whether you can live with that.
How have you been sending your test faxes? I hope that you were using your fax machine and landline, connected directly, not through the OBi. Otherwise, you may have been using a degraded source and were not doing a fair test.