Are you talking about the V7A or the V71A?  The DFK-3C is meant for the older V7A and uses proprietary connectors.  The V71A extension kit is the PG-F5 (an optionally the DFK-3D) and uses standard RJ45 connectors.

The mic extension cable is the same for both radios.  The control head extension cable is different.

That presumes the OP has a V6 Challenger.

I had my new car less than 24 hours before the first NMO hole was drilled.  I have no experience operating HF mobile but for VHF/UHF I'd suggest a single NMO mount, in the middle of the trunklid and a Larsen NMO270 antenna.

I used a G-450A with my MA5B for almost 10 years and had no problems.  The rotor was tower-mounted on a plate and the mast was clamped in a Yaesu thrust bearing, so no vertical weight load on the rotor.

I used a G-450A with my MA5B for almost 10 years and had zero problems.

I have no experience with the Mosley but had an MA5B for almost 10 years.  I wasn't expecting mind-blowing results but it worked fine for my needs.

Keep in mind it's a 2-element yagi on 10-15-20 and a rotatable dipole for 12 and 17.  Don't expect it to perform like a multi-element beam.

I used a Yaesu G-450 for almost 10 years with my MA5B and had no issues.

Not hearing from Yaesu for a month while they've got your new radio is pretty unreasonable.

I've got an Alinco DM-330MV that's been running more-or-less continuously since when I bought it new in 2005.  It powers my Kenwood TS-2000, Yaesu FT-8900, police scanner and XM satellite radio.  Solid DC output, small form, lightweight, RF quiet.  It just keeps going and going!

If I'm reading correctly, I think the OP mentioned that the original author of the software no longer had copies of the software.

A duplexer is not the same as a diplexer.  A duplexer is a cavity filter that is tuned to notch a certain frequency.  They're frequently used in in-band repeater installations.

A diplexer splits or combines signals from separate frequency ranges.  Comet, Diamond, etc. make them for either splitting a multi-band antenna to feed separate, monoband radios or for feeding a radio with multiple outputs into one antenna.

Check to see if the output of your 2M amp adjusts depending on the input power.  You may be able to feed it with, say, 10 watts and have it dial back the output to 100 watts.

I did this several years ago when my club's 2M repeater was on the fritz.  My radio was the "transmit" side of the temporary repeater.  My buddy's radio about 2 miles away was the "receive" side of the temporary repeater.  We linked the two radios via 70CM.

I fed my Kenwood D7A into a diplexer to split off V from U, ran the V side into a Mirage 2M brick then combined the V and U again with a second diplexer to a single feedline to a dual-band base antenna.  The setup worked reasonably well and the horizontal distance between the two radios was enough to prevent desense issues.

Since your radio already has separate V and U outputs, you'll only need to combine them again using a diplexer after the 2M amp to feed a single antenna (if you want to avoid having to install two antennas).

Larsen NMO270 with the black "curly cue" whip.  Not as much gain as some of the Diamond and Comet monsters but it's pretty much indestructible.

You can probably reuse the mic extension cable but the head control cable on the V7A is entirely different from the V71A/D710: one is proprietary Kenwood and the other is RJ45.

One thing to keep in mind with the mic extension cable, the audio is analog and can be susceptible to external noise interference.  The Kenwood extension cable has shielding around the audio line.  Some people have had no issues using regular Cat5 but others have experienced a "helicopter" sound on their transmitted audio that went away when the used the shielded cable.  Your mileage may vary.

When I got my V71A I bit the bullet and bought the PG-5F kit too.  Only complaint is that the cables are disappointingly short (about 13').   I mounted the radio body in the trunk of my 4-door full-sized sedan and the extension cable was barely long enough to reach the control head mounted in the dash.  Come on Kenwood, don't be so cheap and give us at least 15' of cable!

The handhelds seems to be quite cost-effective and actually do the job surprisingly well.

The mobile models coming do not have near the cost advantage over similar radios from YaeComWood.  In this instance for a few extra dollars I'll go with the name-brand.

Aluminum I can deal with as it's still a metal.