The RG58 cables pretty much negated the effect of your antennae! (You can easily prove this by repeating your test using the supplied "rubber duckies" instead.)

LMR-240 is superb microwave cable. I own 2 x 25 foot lengths as a semi-permanent installation in one of my regular venues, but I wouldn't recommend it for life "on the road".

...Aah, the romance of the "strolling player" !   (Especially should it be related to the amazing Johann Sebastian...)

It's really bulky and hard to roll-up compared to LMR-195.

You're "spot on" regarding RF absorbtion by the salt-water-filled carbon-based lifeforms that attend performances, which is why I take duct (gaffer) tape, tall boom stands as well as a variety of cheap cable ties, clamps and mounts in order to easily arrange (and subsequently, easily de-rig) line of sight orientation for my antennae.

Given a variety of locations and venues, I feel you'll need at least one 25 foot length (or, even better, 50 foot) of the much more easy to wrangle (yet very effective and much cheaper) LMR-195 BNC cable that'll enable the true diversity system to "see" around such RF obstacles as crop up. Be aware that trees, shrubbery (even potted) or any metal can cause problems. It's often handy to have the convenience of one antenna near by, with the other coming from the left or right "outfield".

The best price I've found for high quality LMR-195 BNC's (albeit in pairs) is here:

https://www.globalfulfillment.net/gfsnet/(S(k2p52h45a0h5oy2lsw0kb255))/line6/10Expand.aspx?ProductCode=98-033-0017

As for cheap clamps and mounts, I've listed two below, both of which I regularly use in preference to stands as they very often allow mounting that's less susceptible to alcohol-fuelled interference.

A neat trick for the quick overhead (hence, out of alcoholic reach and/or sight) "flinging" of antenna (or other) cable is to use a $2 beach fisherman's hand caster (looks like a plastic railway wheel) to store 15 to 20 metres (45 to 60 feet) of polypropylene "clothes line" rope for a "pull through".

Attach a suitable "sinker" to its end, creating precautionary "shock-proofing" by having it encased in the soft, empty PVC housing from a standard mains socket or some such.

You simply "cast" this sinker and cord over whatever smooth "bracket" or "hook" the location provides and haul away.

Incidentally, one advantage the omnidirectional P360's have is the very fact that they don't need to be "aimed" once they're vertical. It's also often forgetten that paddle antennae are equally effective "BNC up" or "BNC down".

Given the amazing compactness and light weight of Line 6's paddles, this can (in some locations) facilitate ridiculously easy rigging and de-rigging by virtue of simply "hanging" the paddles by their BNC cables. 

http://www.sweetwater.com/store/detail/MicMntClamp/

http://www.sweetwater.com/store/detail/BoomSide/

Regarding the "gain" switch, it has no effect on the antennae themselves, it simply boosts such signals as they're "capturing" to compensate for the attenuation that's inherent in the length of cable.

Accordingly, should the antennae be receiving interference, it'll boost that as well, which is why we strive to orientate them for the best reception of our mics and only our mics.

I tend to use the middle gain position as a starting point for "walk" testing of my 50 foot LMR-195's, which has only occasionally resulted in my use of both higher and lower gain settings, depending on both the XD-V's LED ladder RF indicators and my ears.

Having taken this trouble to orientate their receiving antennae, my TBP and THH12's spend most of their life operating brilliantly in the "battery save" low power mode, as too much RF around the place often creates another set of problems.