It’s fairly unlikely that Susan would have been going to college, either before the death of her family or after. It’s stated in the books that she was not very good at school work, and higher education was by no means as common in those days as it is now; you usually only went to university if you were in the top bracket intellectually, as opposed to today when it is almost the default option on leaving school.
That said, just holding a School Certificate (the exam qualification they had in those days) would have meant she was educated to a good all-round standard that would put her in much the same position as a fair few first-year undergraduates today, and by what I’ve seen on these boards, even Lucy could have taught the average poster hereabouts plenty about grammar, spelling and so on.
The usual thing for a young woman of Susan’s age to have considered would be getting married. Most people did. In her case it would have meant finding a nice young man with an honest family background and a respectable character, with promising career prospects – almost certainly something in an office or bank judging from what I can tell of the Pevensies’ social class – and settling down in her early twenties. In that case she could perfectly well have lived into the 21st century and be a great-grandmother by now.
Of course Susan might have found alternatives. As a beautiful girl who had, apparently, been worming her way into the world of party invitations as soon as she was old enough to pass for twenty-one, she might have found herself a sugar daddy – a rich but much older man to set her up in a nice little flat somewhere with all her needs provided for, in exchange for which he got a young beauty to show off in public and… well, the forum policy requires that if you can’t fill in the blank for yourself, I don’t tell you.
As for her spiritual welfare, it was entirely in her own hands. Aslan may have intended the loss of her whole family to come as the massive kick in the behind that drew her attention to the things that did and didn’t matter in life. I can picture her, all dressed up for a party, receiving the telegram with the bad news, and suddenly everything that she thought was important melting into insignificance as she realized she would give it all up to have her family back for a week, a day, even half an hour.
Or it might have been too much for her to swallow, and she might have decided that if He could let something so terrible happen to her, He could not have loved her as much as He ought to… and she would jolly well pay Him out by not believing in Him.
It was her choice. It’s our choice. It would be well if we chose wisely.