I am about to preliminary conclude the current set-up is now working fine. In summary, two principle issues were dealt with in order to achieve sustained internet connectivity with the RV042 and 2 cable connections from the same ISP; a) set up so the RV042 WAN ports operated through different gateways and b) ensure quality power supply to relevant devices.
On Monday I installed a CDP 750Va UPS and connected modems, routers, switch, ATA and phones. This UPS appears to be adequate for this particular purpose.
See two screenshots attached indicating the RV042's proper load balancing feature; one (1) where a streaming TV station is on and I start a second streaming video, and the second (2) where one streaming TV station is on and I start a speed test. The 'Break point download' mark indicates max download speed on one connection.
Thank you all for great input!