Airport Express Extending non-Apple AP

Question

For something that "just works", the Airport series is a real mess if not every single device you own is the most recent Apple product. Thanks Apple.
Anywho, before complaining too much, I'd like to set my problem.
We currently have a main Airport Extreme (lets call it AEX) that is connected to some switches. We also have 3 Airport Expresses, for extending our network and AirPlay for 3 stereos throughout the house. Now, our outter most Airport Express (lets call it AE3) only receives a faint signal or none at all. So I set up a D-Link DIR-615 (lets call it DL) to work as an AP and extend our wireless network, to give the AE3 a decent signal. Now apparantly Apple managed to only let devices extend a network if the network that it is extending comes from another Apple router. Very nifty. Very user friendly. Since everybody uses all Apple devices. Practical.
Of course D-Link != Apple, so AE3 can't extend the wireless signal created by DL (connected directly via LAN to AEX). However, I can set up AE3 to only receive the signal of DL. Works fine! No extension of the network unforunately - but at least it works.
Now for the real issue - another Airport Express (lets call this one AE2) is also in the zone of DL. When the signal of AEX get's weaker or AEX is updated and offline for a second, it somehow pairs with DL instead of AEX. Since AE2 is set up to extend the network and not only receive - since that's what they are made for - this forms errors.
Is there really no way to let it extend even if the signal they are receiving is from DL? I just can't understand how hard it can be. Since we have already been forced to buy a AEX along with our AE's (since we had no clue a full-Apple-network was required for AirPlay), I'd like to avoid buying yet another Airport Express to make a signal cross the room.
Now that I'm ranting anyway, I'd like to express (no pun intended) my dissatisfaction with following issues Apple is causing.
First off, the Airport Utility for Windows hasn't been updated in a long time. It isn't available on all versions of Windows either. On top of that, the new version requires iOS6/7, thus making a first gen iPad really not that useful for network configuration.
Secondly, my compliments to Apple to not make the AEX support NTFS for external hard drives. BRILLIANT move there. I really do want to use FAT16/32 on a 1TB hard drive, so practical for my large files! No I kid, Mac OS Extended is of course better use for a hard drive that big, surely since all the computers in house are Windows. The joy's of using Apple products never ends does it! I understand they want to keep everything all-Mac, but selling routers one does expect somewhat compatibility with non-Apple stuff.

Answer 1

Is there really no way to let it extend even if the signal they are receiving is from DL?
Unless you want to use a wired connection between Apple and non-Apple routers the extend feature of Apple routers only works with other Apple routers. This is really no different with the other router manufacturers whether it is D-Link, Belkin, Linksys, etc.
(since we had no clue a full-Apple-network was required for AirPlay)
That's not entirely true. One of the requirements for AirPlay is a network, either wired or wireless. The networking hardware that makes up that network does NOT have to be comprised of any Apple products.
For example, since AirPlay requires at least three things: 1) iTunes "server," 2) Network, and 3) AirPlay "speaker," you could have a fully functional AirPlay streaming with the following devices:
iTunes "server": Synology NAS
Network: A wireless network based on D-Link hardware.
AirPlay "speaker: Denon AVR-991