It seems that I need to get a new audio interface because 1) SONAR often crashes when I open a heavy project, 2) when SONAR crashes, SONARPL.exe stays in task manager for some minutes so I can't immediately relaunch the program, 3) a heavy project (which uses 2-3 instances of Kontakt, have about 50+ tracks and 50+ plugins) often refuses to play any sound at all when it's not the first project to open after launching the program (I contacted CW for this and spent more than a few weeks on my side to identify the cause, and it seemed the culprit was the driver of my audio interface Roland Quad-Capture).
 
So I've just searched CW forums, found and read two threads asking for audio interface recommendations, and Apollo Twin USB and RME interfaces were the most recommended. But the first one is a desktop type (which I prefer not to have) and RME ones are too expensive. What I need is a 2-in/2-out half-rack USB audio/MIDI interface of good audio quality that is *rock solid* in SONAR. As for the budget, I don't think I can pay more than $500. I thought the Focusrite Scarlette series would be a good candidate, but one reviewer who uses Scarlett 6i6 G2 with SONAR says (on a Japanese site) that he starts hearing some noise when the number of tracks in the project reaches a certain number (he didn't hear it when he was using Roland Duo-Capture), so I hesitate to consider it as an option.
 
Recommendations from those who have been working with equally heavy (or heavier) projects will be highly appreciated.

Crashes do happen and the midi icon will stay on after some.  The easiest way to get rid of it is to restart SONAR and then shut it down.  A second restart seems to clear it up for me, and that has happened since, well, forever, and across various interfaces.
 
Personally, I've been using TASCAM interfaces for the last couple of years.  The UH-7000 and the US-20x20.  The UH 7000 was their "premium" interface, made with an eye to the audiophile for some reason.  The drivers aren't the lowest latency and they've stopped working on them - or haven't put out new ones in a couple of years.  However, it is superb conversion and clean pres worth the asking price of the whole interface. You can find them for less than $500.  The 20x20 is the later US series (for which they continue to work on drivers for the whole US series [an important fact to ferret out for your long-term peace-of-mind]).  Plenty of ins and outs and, quite frankly, in the same ball park as the stereo 7000 in conversion and similar sounding pres.  Latency on an OK computer via USB 3 is 3-4 ms.
 
Lastly, blaming the interface is usually a fault in the driver and how it reacts to your particular hardware/software configuration.  Might as well blame the computer.  We see the same problem here in CAKEworld.  Despite intense testing, once software gets out to the multitude of computers people use, problems, big and small, crop up.  We just saw a fix added to last month's update because there was a problem that didn't get found in regular beta.
 
Most dealers will let you try an interface and return it if you have a problem.  Find the cheapest interface that meets your specifications, see if it works, and if it doesn't, return it and move to the next.  And really, I haven't heard a modern interface in the last few years that doesn't sound good, and see the above TASCAM units.  One costs $200+ per channel, the other $20, and there is little difference between the two for the most part, sound-wise.  And what I mean by that I could do the work I do on either unit, even if I do most of my work (and monitor through) the 7000.  Might as well get my money's worth for that 5% extra.

I've been using a Focusrite Scarlett 2i2 for the last year and a half.  I've never had a crash, or any problem that looked like it was interface-related.   I'm very happy with the product.

There are other users here running the Quad-Capture without major issues.
 
I don't know what troubleshooting steps you've taken, but will assume you've done the usual things like turning off wi-fi, USB power mgmt, and all other background tasks. Also, that you are connected to USB 2 port (not USB 3)...
 
I see you have two laptops, both of which are running Windows 7.  It if were me in the same situation, I would upgrade one laptop to Windows 10 and see how the Quad-Capture fares then. 

The PreSonus AUDIOBOX USB does what you want and is simple to operate and has excellent mike preamps (2).  Very reasonably priced at around $100.  Small footprint and well built little case.

Whichever way you go, consider buying used. You'll get more interface for your money, and it's unusual that used electronics of recent vintage have issues. You might even be able to step up to RME.

Another thought.....re:  "SONARPL.exe stays in task manager.."   In Windows 7  hit ctrl-alt-del.  Then go into Start/applications and click on the Sonarpl.exe to END.  It will return you to the Windows 7 screen.    Sonar will start then just fine.
 

The Steinberg UR-22. About 99.00.

I'm with AT on the TASCAM US-20x20. RME might be considered the 'IBM' of interfaces but is not necessarily the best bang for the buck in all cases. I'm checking one out now in a small project studio and other than some adjustments to buffer settings it's been holding up well. I was keeping an eye on issues reported with another model in the series (US-16x08) but they've been good about updating both the drivers and firmware. Only time will tell though and that's where RME has rightfully earned their reputation. However, the difference in price with the RME is one or two pretty good mics. 
 
In addition to the previous VST suggestions, the issues you mentioned sound like they could be related to your PC config. I've used the Quad Capture with a laptop to track some audio (no Kontakt-heavy projects) and while generally OK, did have its driver problems every so often, requiring a reboot. 
 
hth. 

Steinberg UR44 or UR242.  Rock solid construction and drivers (made by Yamaha).  If you need mobility, then the UR22 MkII.  (Personally, I stay away from bus-powered interfaces, but then again I don't need mobility). 
 
I've heard one well-known DAW builder say that RME and Steinberg interfaces get the least amount of reported issues at his shop.  Years of forum reports from RME users certainly bear that out, and as a longtime owner of Steinberg interfaces (I own the MR816X and UR44) I can absolutely believe it about Steinberg.