I think it's all relative to your gold usage, how much more science you plan to add in the next dozen turns, and whether you have any bonuses to RAs (from policy or wonder).
There are no hard and fast rules or answers to your questions. (Except for your question #2, that doesnt' matter anymore).
If your science output grows at the same relative rate as the costs of technology, then an early RA would be worth the same as a late RA. In fact, an early RA would probably be worth more relatively because the AI has a larger tech gain advantage earlier on, and you catch up to them by industrial/modern.
I also never sign RAs with the runaway (even if we're friends). There's no reason to help them win any faster.