There aren't many here that will be able to help with this, but I might be able help with some aspects or trigger an idea or two by describing what I do.
I play a JTV-69 (VDI) and (second) keyboards in our Prog Rock band, and my keyboard sound is routed through my HD500 so we have some similarities, but there are also differences which will limit how much advice I can give:
1) I don't have a DT25 - instead I use the XLR outs to either 1 or 2 FBT Maxx2a FRFR powered speakers that can be backline or monitors with XLR daisy chained to PA if the gig is big enough. So I can't help with the DT25 specifically
2) I don't have a Mac or MainStage - instead I have an ASUS laptop (touchscreen) running windows 8 and I use Bloxpander as a VST host. So I can't help with any specifics on the software side.
For keyboard heavy songs (Watcher of the Skies) I have a dedicated Keyboard patch, for songs which are guitar only (Aqualung) I have a dedicated Guitar patch that gives me the 3rd fret virtual capo on the acoustic patch. For other songs however such as Echoes I play part Keyboard and part Guitar using a single patch.
My keyboard is an old Korg N364 which I primarily use just as a controller, but also have patches on it where I haven't yet got a VST equivalent so it is part primary instrument and part backup should the laptop fail.
I take MIDI Out from the N364 into the MIDI In on the HD500 and then set the HD500 to MIDI Through - so it merges incoming MIDI with internally produced MIDI and sends it all out of the HD500 Midi Out/Through which is connected to the MIDI In of the Laptop (USB Midi adaptor). The Laptop MIDI Out is connect to the N364 MIDI In, but generally nothing is sent down this. The N364 doesn't pass MIDI In through to MIDI Out so there is no loop.
Audio signals go as follows: JTV into HD500 using VDI, N364 in stereo via two 1/4" Jacks to HD500 FX Loop Returns. HD500 XLR outs to FRFR speakers.
What about the Laptop sounds? well that is USB - ASIO drivers on the laptop send the sound directly to the HD500 Audio Outs - however if this doesn't work Plan B is 1/8" headphone out on Laptop split to 2 x 1/4" into the FX Return and Plan C is 1/8" into HD500 MP3 input.
So how do I control Keyboard Volume?
There are two methods depending on which audio feed I have:
a) If it is the N364 feed I have the option of controlling the FX Return level with the Expression Pedal (or can preset it on the patch), you could add a volume pedal after the FX Loop, but the FX Return is just as easily controlled as the Volume pedal, (method b also works here just as well)
if it is the USB Audio feed I have no control within the HD environment as the USB Audio is fed in after the HD Audio processing (only the Master volume knob works), but I simply control the volume by setting the expression pedal on the HD500 to transmit MIDI cc07 data (volume).
This is on channel 16 against which the master volume of the VST host is linked - the individual sounds are on channel 1, so basically Ch 1 is used to control the Instruments including sometimes cc07 to crossfade between sounds, while Ch 16 is used to control the Host.
I use Ch 16 Note messages to change sub-scenes in the VST Host and at the moment only have a single Patch (Scene) but control which of the VSTi receive which aspects of the MIDI on Ch 1 by switching between predefined configurations in the Host. This sounds complex, but basically the VSTi are all active all of the time, I just switch the MIDI routing around them to define what VSTi gets sent the Note On messages at any time - they all receive Note Off messages all of the time which is important if you want to avoid the hanging note problem.
The Note messages to change Keyboard sounds could just as easily be Patch Change or Continuous Controller messages and they are sent by adding MIDI messages to the foot switches on the HD500 - I set the Host to ignore the HD500 generated Patch changes because I don't want the keyboard patches to change as I change guitar patches,
I use the top row of switches on the HD500 to switch on and off Guitar effects, the bottom row to switch keyboard patches and have the HD500 in pedalboard mode so I have to hit the up/down switches to change patches. I also have a second Expression Pedal connected which acts as the main volume control with the Expression 1 controlling effects. The latter can mean that when I need a big keyboard fade in or cross fading between sounds, that I have both expression pedals controlling Keyboard volume - but one controls the master volume of the VST Host (my main level) while the other controls the volume of the individual VSTi instruments.
Back to your questions:
Recommended amp for the keyboard? I usually default to the Blackface Twin, but I'm not quite sure it's the sound I want. I know I can't use NO amp model if I'm outputting thru the DT25.
I honestly don't know - I do sometimes put the N364 through an Amp model, but that is mainly a clean Hammond sound into a Marshall in order to get that authentic Deep Purple distorted Hammond sound. Pick the cleanest and flattest that you can - you could try the Hiwatt which might not have been an obvious choice, but was intended to be very clean when it was first produced.
It may be that the FX loop trick placed after the Amp model (so the Amp model has nothing going into it) will work quite well as this avoids any Pre-amp modifications to the Keyboard sound.
Will I get interference issues when playing guitar if I turn down the keyboard volume, and vice versa?
You shouldn't - the inputs into the HD500 are isolated from each other and you can select which inputs are active at any time on a patch by patch basis.
Is there another "best" way to do this? Maybe plugging the keyboard into the DT25 directly will give a better sound, although it would stop the volume pedal from working?
Using MIDI will allow you to control the volume this way, other than that I wouldn't say that there is a "best" way, just a whole load of options that you might try to see what works best for you.