RE: On Board Data Logger
I've never understood leaving the data logger in any plane. If you only plan to own 1 plane than I could see the reason. I own several planes and I do this.
Ernst and several other companies sell Empty charge ports. These small units will fit the male end of a servo wire. Make a male to male extension. Attach one end to the mount and one end to the RX in the data port. Now you can mount the charge port anywhere on your plane you wish. On my GS IMAC planes I mount them on the outside so I don't have to take the hatch off to view the data. On my jet they will go on the electronic tray next to the on/off switches.
For the data logger you make a female to female extension. That way you can plug into the charge port and better yet you still have a standard Data Logger that a friend could borrow and plug directly into their RX if need be.
Now for the cost of about $3-5 I can put a data logger in every plane. I have a pic attached of it installed on my 35% Su-26.
As for running a battery Y'd into that port I think you are mistaken. The data port isn't a battery port. That would be the Bind/Battery port. I'd Y into a different port than the data port if it was me.