An Extraordinary General Meeting of the WRU has been called by member clubs amid uncertainty over the plans that will impact Cardiff, Dragons, Ospreys and Scarlets.
The governing body intends to have one club in the east, one in Cardiff and one in the west, sparking turmoil, protests and legal action.
WRU chief executive Abi Tierney has admitted she “does not feel safe” in her job due to social media abuse and Jewell said the committee recognised WRU bosses had faced heavy criticism that was “unduly harsh”.
But the Plaid Cymru MS added: “The WRU must recognise that it has lost the argument over the future of the professional game.
“We were alarmed to hear you [Collier-Keywood] tell us that you have no plan B should your current proposals not progress as planned.
“Were this the case, it could represent a significant dereliction of duty. We urge you to find an alternative approach that continues to stabilise the finances of the Union and maintains confidence, whilst rebuilding trust with clubs and supporters.”
Last week Collier-Keywood admitted the governing body had struggled to bring the professional sides on board with their plans.
The WRU chair, who appeared along with director of the community game Geraint John and senior independent director Alison Thorne, maintained that financial struggles meant that a three-team model was the only way forward for the game.
“The people of Wales feel a deep sense of ownership, pride and emotional connection to the game. That connection must be respected and nurtured,” wrote Jewell.
“The Union must not find itself in the position of winning the vote but losing the soul of Welsh rugby.”