Welcome to the forum.

Your only real options are to use a faster lens and/or faster ISO. The latter increases the risk of digital noise in the shadows. The K20D produces less noisy images than the K10D at higher ISO settings.

About the fastest Pentax lens you'll find are the 50mm 1:1.2 lenses in various forms. The A version is to be preferred but the older K version is also very good. You'll not run into the metering/exposure issues that are well documented elsewhere here, because you'll presumably use the lens wide open at f1.2. The FA 50mm 1:1.4 is a good option too, cheaper, and only half stop slower than the f1.2 lenses.

Welcome to the forum.

The Pentax Xtra Flash is a slave flash. It is triggered by the light from another flash, like the onboard flash. The problem with it is that it is not pre-flash aware, which is what the onboard flash uses to determine flash output for correct exposure.

If your Jessops gun does not use a pre-flash system, just turn on the Xtra flash and site it where you want it to contribute light to the scene. When the main flash fires, the Xtra flash will also fire.

No, it doesn't.

Use a shutter speed anywhere between 1/60th and 1/180th, it'll not make much difference as the flash of light from a flash gun is around 1/10000th of a second and sometimes as fast as 1/40000th. Much faster than any camera shutter. It's the aperture setting and flash to subject distance that affects the exposure most. Start with f11 and take it from there.

To be honest, you're better of finding a book on flash and/or basic studio lighting. It's such a huge subject on its own, it'd be difficult to answer your question here.

EDIT: It looks like your flash may be TTL only, in which case it'll probably fully discharge (maximum light) when you use it with your camera. TTL flash was supported on the *istD and *istDS only and as it's a Canon flash, is unlikely to work properly even on those bodies.

How about this ?

Look out for either a SMCP-F 100mm 1:2.8 Macro, or the SMCP-FA version. You'll not be disappointed by the results.

Both of these have a FF image circle, the D-FA version should be good also.

AF160's are cheap but to use it as you intend, it will always contribute light to the scene.

A better solution is to choose an older flash that has tilt/swivel capabilities. When the head is pointed straight up, your subject will not be illuminated by direct flash and your AF360/540FGZ will still trigger.

All this assumes you are inside so that some bounced flash is received by the AF360/540FGZ to trigger it. This may not work so well outside in an open space.

SRS Microsystems will usually trade, but you're better off selling & buying separately.

A video Podcast has to be the way to go with this Matt.

I owned a Tamron SP 500mm mirror lens, arguably the best of the bunch (other than Pentax), for many years and seldom used it. I've since sold it and much prefer to use shorter focal length lenses.