yes. because 'meron' is the short term of 'mayroon'

whatever suits you, really. contemporarily though, "meron" is likely used. "meron ka pa ba'ng tanong?" (do still have anything to ask?) :)

Yup. Mayroon = meron also as Kaylan or Kailan = kelan But it's more polite to complete the word as Mayroon. Meron is a slang and used only in verbal but in a writing much better used Mayroon. Also, meron can mean both "have" and "there is" right? - Sometimes you can use have = meron as example: Do you have a car? = Mayroon ka bang kotse? but not always depends on a sentence. Example: Have you been here before? = Nanggaling ka na ba dito? (asking if you're already been here) Have you eaten your dinner? = Tapos ka na bang kumain ng hapunan?

Yes. Mayroon is the proper word to say, spell and write. Meron is a slang and not included in the filipino dictionary. This is commonly used by people who are talking, texting etc. but should not be used in proper writing. Anyway, in your example, Mayroon akong ng kotsche. and Meron akong ng kotsche. (It should be Mayroon akong kotse. Meron akong kotse) Hope that helps!