Bill Fitzmaurice

Allison Effect, the cancellation from the reflection off the rear wall. To prevent it the subs must be close to the wall, the mains far from it. When both are the same distance away it's unavoidable. See above. Since you can put the sub anywhere that relieves some of the space concerns. By dint of using a bass combo as a sub it is a sub, and the same placement rules apply.

Because the sub probably has more available output from 60-100Hz than the tops. Crossing lower than 100Hz with tops that are on stands also brings floor bounce cancellation into the equation. No, as that results in directionally locatable frequencies coming from the subs. Then you lose the ability to put the subs anywhere, which should never be below the mains unless there's no alternative.

Once again, port shape ...doesn't matter, though narrow slots should be avoided. That's as far as sound is concerned. From a construction standpoint I use corner ports, as they do double duty as bracing for the baffle, top, bottom and sides.

That being the case you don't need the subwoofer bottom cab of a Bose clone, even as meager as it is. You do need two speakers, for even coverage. Size being the issue a pair of powered 8" or 10" loaded mains will do, and you'll get a lot more value for the money with those than with mini line arrays, which tend to be terribly over priced. Another advantage is you can use them with your existing sub when the need arises, and add them to your regular system for outdoor gigs.

Bracing is seldom unnecessary, unless you're building with 36mm plywood. My personal cabs are built from 3mm and 6mm plywood, which is possible because they use necessary bracing. Sure, if you want state of the art circa 1984. 😲

Port shape (duct actually, the name 'port' is incorrectly applied to ducts, but it's part of the lexicon now) doesn't matter, though narrow slots should be avoided. The area of those ports looks to be far too small.

It would also result in chuffing, as the port area would be too small.

The cabinet is simply far too large, so it doesn't provide the necessary restorative force to prevent over-excursion. That mainly relates to the driver Vas. For the K140 that's 297 L. For the 3015 it's 153 L.

That diagram shows the 3015LF, not the 3015. The 3015 works best in 70 liters/2.5 cu ft (net) with 200 cm2 port area 35cm long for 45Hz tuning.

In 60 years of playing I never had an amp failure, so it wasn't worth it to me to carry another piece of the heavy iron that ruled for the first 40 years or so. But given how small good amps can be had today if I was to keep a spare it would be one of those midget amps rather than a pedal.

Without detailed specs on the woofer and tweeter one cannot construct an ideal crossover. If you have the impedance of the drivers you can use an off the shelf crossover that will still be better than what you have, for instance https://www.parts-express.com/Eminence-PX-BII-3K5-2-Way-Crossover-Board-3-500-Hz-290-636?quantity=1

The APT drivers were changed years ago from a proprietary non-standard attachment to a standard 1 3/8" screw on. The horn in your picture looks like the old design. A picture of your driver would confirm if it's the new design.

Hum is almost always caused by grounding/earthing issues with the wiring and there's seldom anything you can do about it.

That's not a crossover. A crossover splits the low and high frequencies, sending the lows to the woofer, the highs to the tweeter. That's just a high pass filter, and a very poor one at that. It's what's called 1st order, meaning that it attenuates below its knee frequency at a rate of 6dB per octave, which is totally inadequate. I never use less than 3rd order high pass filters, which attenuate below the knee frequency at a rate of 18dB/octave. That makes them over ten times more effective in reducing low frequency content to the tweeter that leads to high distortion and blown tweeters.

Search 'sound analyzer app'. I can't recommend one as I don't have iPhone.