I suspect that even Kwendrith was not sure who the father was, she was compulsively promiscuous according to the script.

It is also ridiculous to portray him as crippled. The term "boneless" can mean many things in that era, not necessarily a cripple.
No cripple could have been a great warrior in that era, magic chariot courtesy of Floki, or not.
The second he hits the ground he is nearly helpless during a real attack.

Nonsense. Likely she was about 16 when she wed Ragnar, Bjorn, her first born, is likely about 29, making Lagertha about
45, hardly ancient.

All true. Remember how she killed the Queen when the King tried to kill Ragnar and his sons - that was a fellow shield maiden, experienced and strong, and Lagertha had no problem besting her.
Three young boys, one a cripple, would not have been able to kill her.

She is also quite capable of taking on all 3 of the brothers who went for her. I thought it improbably and unnecessary for Bjorn to "save" her.

You are totally wrong about Norse women not being allowed to rule. There is historical evidence that Lagertha
was a "Jarl" and came with ships to help Ragnar take back Kattegat from Jarl Borg.

Biggest stupidity was Bjorn showing up just in the nick of time to stop Lagertha vs. the Ragnar sons - the plane must have landed just in the nick of time and Bjorn got a message on his cellphone.

Also some historians think that "boneless" meant impotent. There is some possibility that he was a sexual sadist, not capable of normal sex.

There is also the fact that Norse culture did give equal rights to "free" women.
They had the same right as men to voice their grievances, opinions, and demands at the "Thing".

You are not supposed to "like" Ivar - he is a sadistic psychopath.

Bjorn may well be Rollo's son ——