Read your other post with details about your setup. You provided a lot of good information, some of which may have clues as to the issue.
"I've moved from an older Macbook (late 2011, OSX 10.10.5, continues to work well with the home network) to Early 2015 Macbook Pro 13" (256GB, Yosemite 10.10.4. The problem shows up after a) using Migration Assistant to install only Applications, and b) OS is updated to 10.10.5. Migration Assistant transferred only Applications from an older time machine backup of the older 2011 Macbook"
This indicates the issue might not be with 10.10.5 per se but rather with the setup (or hardware) in the newer computer. Use of Migration Assistant means you may have migrated over something older you had installed that is not compatible with the combination of hardware/firmware you are now using. This is one of the conundrums with the "brand new laptop" WiFi failures some are reporting, as the process of migrating old stuff over brings in the possibility that some of that old stuff doesn't work smoothly with the new hardware/firmware combination.
"Immediately after using Migration Assistant to transfer applications, updating to 10.10.5 I was able to observe the problem again ... Connectivity at work is 100% stable, even after Migration and Update with the third and the original Macbook."
That makes me suspicious of your router combination at home since the new laptop works well with routers at work.
"My home network:
* Belkin Router N600DB v3 (firmware up to date).
* Its setup with DHCP "off" to allow my Macbook to setup IP address from another router which connects to my ISP.
* pre-configured DNS entries.
* Wifi is 5GHz, WPA2 (AES).
* All channel selection is Auto."
Your combination of routers sounds interesting. Can you try a temporary more simple direct router => computer arrangement? The fact that your laptop is fine at work sounds like it works when some router combinations but not with others. I'd scrutinize the home setup. Also, just an anecdote, my home wireless connection is not stable when channel selection is on "auto" -- so I checked other signal strengths and picked the channel that looked best and told my router to use that one. Might not work for you but I'm offering that as something to try. Also check if there is a channel bandwidth setting you can adjust.
"So far I've learned, its not Wifi interference: signal strength is 80%+. None of my neighbours are using 5GHz band and there aren't any walls in the way."
Signal strength reflects both the wanted and unwanted signals on your channel frequency, so that's not a good indicator here. 5 GHz is much more fickle than 2 GHz and interference can come from other RF sources than WiFi signals, appliances etc.