As I know you'll appreciate, suffering often yields benefits that seem impossible at the time...

In this case, you having suffered this problem will almost certainly help others, as the details you've set out in your excellent post highlights four issues that have given many of us grief.

These are:

1. While a  "daisy-chain" of up to six receivers is fine when using the V70's, (albeit always with  a pair of their supplied terminating plugs attached to the antenna "out" BNCs of the "last" receiver in the "chain") "the self-terminating" V75 is normally limited to a "daisy chain" of no more than FOUR receivers. ...So (in the absence of more cash spent for the extra flexibility of an antenna distribution unit) the best way for you to connect your rig is to ensure that your sole "self-terminating" V75 is not only the LAST in the "daisy chain", but also that it has NO V70 terminating plugs attached to its "antenna out" BNC connectors. (I frequently operate my sole V75 like this, "hanging off" and terminating one of my two Gator-cased racks of four V70s that are daisy-chain fed from a single pair of P180 antennae);
2. V75s (like your "offending" TBP body pack) all arrive set to operate in the newer "more WiFi-friendly RF2" scheme of frequencies, whereas V70s originally came with access to only the more robust (but a bit less WiFi and Bluetooth-friendly) "RF1" scheme. The important thing to note here is that any attempt to "force" simultaneous operation of BOTH schemes in the same venue WILL undoubtedly result in an RF disaster. Re-tuning details are set out here, http://line6.com/support/docs/DOC-2645 and a discussion of the relative merits of both schemes can be found here: http://line6.com/support/message/374285#374285;
3. The WiFi remote operation of your excellent Presonus system may well influence your choice of RF scheme, but remember that you'll need to have it (and any other "local" WiFi or Bluetooth gear) "up and running" at the time of any "Channel Scan" for such a scan to yield viable results and
4. If you haven't already done so, ...it would be a great idea to use your XD-V75 for a "re-flash" of your four V70 systems (both transmitters and receivers) to the latest "75-like" firmware, which would grant your entire radio mic system maximum flexibility. The "here's how" of "re-flashing" via Line 6 "Monkey" can be found here:http://line6.com/support/docs/DOC-2585.

That's a lot of reading and checking, but it's all in a good cause !

So you have 14 units running simultaneously which means that you are running in RF2 mode.  Do you have any Wi-Fi running in the venue?  Typically you will only be sucessful with 14 systems if there is no Wi-Fi.  If there is a Wi-Fi channel running then it is likely that you will be able to use 8 systems in RF2 mode.  You will have to scan to see if all channels are open and available.  Depending on where the WAPs are located you may be able to use antenna distribution and careful antenna placement of paddle antennas and get 14 channels running.

Alternately you switch over to RF1 mode and run 12 systems (but you cannot also run the others in RF2 mode at the same time).

Cellphones are probably not much of a problem (unless thay are also running Wi-Fi at the same time).  Cellphones can get into the lav mics and their cables and become part of the signal before entering the transmitter so segregating them is always a good idea.