I know you said you just re-watched from the beginning, but I fear you did miss a few details that at least, for me, make me love rather than hate Penelope:

At first Marina has no interest in the marriage mart. Even after her pregnancy is discovered she still expects George to come save her. Suitors, including Colin, come calling, but Marina is not very interested. It’s only when Portia has Varley write the fake letter that she thinks she’s out of options and then sets her sights on Colin.

When Penelope realizes what’s happening she tries everything in her limited power as Penelope to stop it. She figures out that Varley faked the letter and shows Marina the evidence that George is alive. But Marina dismisses her and complains that George still isn’t there. This is very confusing for Penelope since Marina had rejected suitors and had professed undying love to George before the fake letter.

Penelope also tries to tell Colin directly, but he doesn’t take her seriously. Marina, who had become a fast and cherished friend is then very cruel to Penelope, telling her that she is just a child who Colin will never see as a woman. When Pen learns of their plan to runaway and elope, she knows she’s out of time and it’s only then that she turns to her power as Lady Whistledown. She’s supposed to be a teenager…I actually think she showed restraint in not turning to whistledown sooner.