1. When can I start feeding baby rabbits greens, if ever?
If they haven't been on it while they were with mum I would wait until their guts are more mature. For me it age would depend a bit on breed/size (as they may mature differently) and if there've been other recent stressors in their lives. If they got it with mum and you know what they typically got I'd feel okay with continuing this.
2. Should I give them unlimited pellets? I know this is a debated topic, both opinions are welcome.
I was told by our vet (dedicated rabbit vet) to give pellets ad lib while they are young and growing, so long as they're also eatting appropriate amounts of hay well. If I had a very picky eatter who wasn't touching hay and just stuffing their gob with pellets I might rearrange the feeding schedule a bit. Luckily all mine pounce on fresh hay with the enthusiasm I pounce on chocolate with. LOL
3. Can and should I feed them straight alfalfa pellets in addition to regular pellets.
I only use timothy pellets as this is current advice from our vets. If your buns don't have issues, aren't overweight and are eatting their other foods in appropriate amounts, a moderate amount of alfalfa pellets mixed in with the timothy is probably not likely to harm them. Obviously if you saw problems or your bunny has had issues, has family members with issues, is overweight or eatting the alfalfa to the exclusion of appropriate amounts of hay then common sense says to talk to your vet and reconsider things.