It is said that if you are submitting your novel to a publisher, use their guidelines for formatting. I have written an 80,000 word novel, does this mean that every time I want to submit to a different publisher I have to rewrite the 80,000 words?
Not rewrite, re-format. This includes spacing between lines, indent size, margins, font, etc. The actual text can remain the same (except maybe if one publisher is e.g. British and the other American, since expressions and spelling differs a bit, as well as double vs single quotes for dialogue).
No. Absolutely not. Changing the manuscript to match a publisher's formatting standards should take you maybe an hour or two, probably much less. That's IF the manuscript is already a reasonably standard format. If it's not, you may have to spend a day or so cleaning it up so that it is. This is reminding me of your question about page wrapping. Both questions make it clear that your knowledge of word processors is extremely limited. I very strongly recommend that you grab a friend to give you some lessons on word processors, or that you take a short word processing class. You don't need to know all the fancy stuff, but you do need a base level of knowledge. Right now, you're treating a word processor like a typewriter, and they are very different.
Yes, this. It takes me about 30 - 45 minutes for me to get my manuscript reformatted to my publisher's specifications before I submit.
NOPE! Just do a global change to tweak the formatting. Most formats are pretty similar. Changing to meet a publishers requirements should not take long at all.