I have spent the last 30 minutes scouring the Internet for things related to this and it seems as though it was decided as Sherry Bilsing and Ellen Plummer decided to create Ross as the father at some point between the end of Season 7 and the start of Season 8. And I am really annoyed with myself that this is the best I can do. :(
It seems as though this is one of those questions that won't get you the result that you want. Writers on shows don't commonly given interviews about why they did what they did as people are more concerned with the actual show rather than things like this. The only hope you might have is that someone has the DVDs and it has some sort of commentary by the writers. Even so it doesn't seem as though that is something that they would put into the DVDs.
As for your option of Rachel becoming a single mom, in the early 2000s there was more stigma surrounding single mothers. For example, this:
In 2016, 29% more children grew up in single-parent households compared to 2000 or 1980. The negative attitude towards single parents is diminishing.
was taken from this parenting site. If Rachel had become a single mom, it could have lead to a drop in viewership (which leads nicely onto my next point).
If we were then to assume that Rachel would have someone to help her with the baby, who else would be better than Ross? He is someone who has been attracted to her since college/high school and is very intelligent. Other people such as Joey would be a nightmare compared to him. I mean, can you imagine Joey raising a child? And if they decided to go with someone new or previously unimportant, fans would ask the exact opposite to your question: "Why didn't Ross and Rachel end up together?". And angry fans lead to fans stopping watching the show, which leads to a drop in viewership, which leads to a loss of profit. So it makes sense for Ross and Rachel to end up together.
No. No-one has said anything about this (AFAICT) which is a real shame. However...
- Rachel wouldn't have been a single mom as society wasn't as accepting as it is now
It makes sense that a member of the main cast would be the father:
- Chandler is married to Monica and it just wouldn't work
- Joey is the kid in the relationship. He would be an absolute trainwreck!
- Therefore, Ross is the only one left. Plus they already have chemistry and have dated before.
Other non-main cast members wouldn't have been appreciative of Ross being shunned by the writers, especially given that all the main cast except Ross would get a happy ending.
I hope this answers your question and now its time for me to take a real hard look at what I'm doing with my life (this took 30 mins to research before saying enough and 30 mins to write and grammar check. I am dead.)