Curate, connect, and discover
In your opinion, would Princess Viserra's fate have changed if she had been promised to Lord Manderly's heir, and not to him?
That is to say, if marriage to the Manderlys were necessary to formalize a pact with the North, perhaps a handsome husband of the same age who would make her Lady of White Harbor would be more interesting to the young princess.
I won't hold back. NOTHING about Viserra's storyline makes any sense short of all the characters having a stroke.
Perceptive and compassionate Alysanne, who previously demonstrated considerable matchmaking skills, suddenly becomes very bad at it. (Don't get me started on Elio Garcia's attempt to defend Alysanne's OOC behavior as her being jealous of her own daughter's beauty.)
Jaehaerys suddenly becomes an absentee father from the 1950s rather than, you know, a medieval king for whom his daughter's marriage is of the utmost political importance.
Septon Barth? Grand Maester Elysar? Both MIA.
Theomore Manderly? If he was truly canny, he would have seen the offer for what it was and declined it.
Its true, the best way to tie the North closer to the south (and more specifically the Iron Throne) short of House Stark is House Manderly. However, marrying Viserra, a princess (and the prettiest one at that) to Theomore entirely defeats that purpose. One, a fat man old enough to be your grandfather is hardly appealing on a personal level, especially to a teen princess who knows just how attractive she is. Two, any children Viserra had would come behind four wives' worth of stepchildren, practically guaranteeing that the future Lords of White Harbor won't share any direct blood ties to House Targaryen, which brings me to my third point. Marrying Viserra to Theomore more or less dooms Viserra's line to obscurity and she herself to a life of dependency on the charity of her stepson.
What, then, would have made way more sense? Marrying her to Desmond, who, as Theomore's great-grandson, would probably be fairly close in age to her, not to mention higher in the succession.
Now, to answer your question, yes I absolutely do believe Viserra might have lived a longer (and perhaps even fulfilling) life if her parents had picked someone other than Theomore Manderly to be her husband. Remember, the whole reason she was even out and about the night of her death was because she wanted to have fun one last time before resigning herself to her fate. If she had instead been looking forward to her wedding she wouldn't have had any incentive to sneak out, which makes the chances of her suffering a tragic accident the way she did in OTL astronomically lower.
Thanks for the question, @diegoedil