They should have left Ellie a widow

[Snarky Oracle!]

Is it known whether the producers intentionally gave Miss Ellie a new husband, or whether they decided to do so because they realized there was good chemistry between Miss Ellie and Clayton?

Was there actually an alternative to Clayton as Ellie's second husband? I only remember one other admirer of Ellie's, whose name escapes me; Miss Ellie convinced him to abandon his construction project. Could he have been the right one for her?

Did the writers ever express regret over Digger's death, as a romance between Ellie and Digger could have been appealing after Jock's death?

Keel thinks they introduced Clayton with the knowledge that Davis was dwindling, and the intention of potentially pairing Howard up with Miss Ellie.

Frank Crutcher was tossed out there as a possible, but Dale Robertson was afraid of homosexual horses and would have baled the moment Jack Ewing showed up.

Keel was the correct choice. And Leonard Katzman was never going to let Mama chew out the cartel again after she did so in JR's office. If you were a woman who lived under the roof at Southfork, you couldn't have an edge unless you were drunk. So Miss Ellie whimpered into her husband's armpit.

Duffy once claimed, quite unnecessarily, that Pam's goodness was "secondary to Bobby's goodness"; Howard Keel said Ellie got her strength "through her men." Misogynist conditioning from Uncle Lenny?

At least all the other Ewing women eventually came in to their own. Except for Miss Ellie -- and when she was needed to be at her most present and proactive in the closing seasons, she was just further sidelined. (A bad sign of how tone-deaf Katzman had become).

So Mrs. Claus was relegated to just packing Santa's sleigh -- until she eventually fell overboard once and for all.

 

[Jock Ewing Fan]

Is it known whether the producers intentionally gave Miss Ellie a new husband, or whether they decided to do so because they realized there was good chemistry between Miss Ellie and Clayton?

Was there actually an alternative to Clayton as Ellie's second husband? I only remember one other admirer of Ellie's, whose name escapes me; Miss Ellie convinced him to abandon his construction project. Could he have been the right one for her?

Did the writers ever express regret over Digger's death, as a romance between Ellie and Digger could have been appealing after Jock's death?

I always thought that it was a mistake to write out Digger, for the reason that you mention, and it closed an interesting possibility.
I don't know if there was any good way to proceed, once Jim Davis passed. It was not the same show, the balance/dynamics/interactions of the characters changed, and Ellie became little more than a plot device. Howard Keel did as well as could be expected, but it just was never really as as good IMHO
Seasons 5-10 (except the dream season) were still mostly strong , but the cast changes seasons 11-14 have been discussed
at length, and the result was a declining show