I haven't read much detail about alchemy in ESO, and I'm just wondering if it will be a viable healing source, much like it was in past Elder Scrolls games, or if healing will need to be done by magic/staff?

Knowing about alchemy and it's healing potential, would greatly help me know if I could make a sword+shield character who uses archery as backup, or if they need to invest in healing staff skill instead of sword/bow. In Skyrim, I love playing as a ranger character who uses both sword+shield and bow, with alchemy potions for healing. Depending on the situation, I usually start as a stealth archer, then switch to sword+shield when enemies are within melee range.

I don't think you'll ever find an MMO where spamming health potions is a viable form of healing. No, we're most likely going to see restorative potions be USEFUL, but not good enough to carry you through stuff. Likely there will be buff potions and poisons and the like to make for the most part.

It would be nice if I could simply "import" my Skyrim character, but I understand that spamming health potions would be bad for a mmorpg. Hopefully, crafting potions remains a good source of income, at least.

Consumables are almost always a lucrative time investment, fear not.

It would be nice if they had diminishing returns to where you could use more then one.

but not to the point where it was too much

From experience the only alchemy proffesion that actually worked was in Ultima Online. Other than that they have been money sinks with no real use. As someone who wants to mix their own poisons to use on the arrows and daggers I really do hope it is a decent proffesion.