Some of these may seem pathetically easy to some of you, but I'm wanting to make sure I have these absolutely correct

1) How long after the events in Morrowind does the Oblivion crisis happen?

2) Is it feasible for the Nerevarine to be the playable character in Oblivion? I know the npc's talk about him having disappeared...but as regards timelines, age, pesky interventions by unnamed deadra again etc.

3) I seem to recall in Morrowind, Vivec saying that the playable character wouldn't be able to connect (and therefore make himself a God) to the Heart of Lorkhan. How exactly did he do that, and would it be possible theoretically to learn how?

Sorry if these sound stupid questions to you, but I'm very curious about the answers - and it would help me a lot to know. Thanks for your time

1) 6 years. MW takes place in 3e 427, OB in 3e 433
2. Not likely. The Nerevarine was immortal and immune to all disease. The Champion of Cyrodiil was not.
3. Perhaps. But he would only be able to do it once, since the only way to stop Dagoth Ur is to remove the enchantments from the Heart.

1) How long after the events in Morrowind does the Oblivion crisis happen? There is an exact date, but I never payed attention to either game's fictional calender. Someone will be here with the answer shortly.

2) Is it feasible for the Nerevarine to be the playable character in Oblivion? I know the npc's talk about him having disappeared...but as regards timelines, age, pesky interventions by unnamed deadra again etc. Only with an Argonian. Otherwise, you have a notable continuity error: Your character is not immune to disease.

3) I seem to recall in Morrowind, Vivec saying that the playable character wouldn't be able to connect (and therefore make himself a God) to the Heart of Lorkhan. How exactly did he do that, and would it be possible theoretically to learn how? The player could not use the Heart of Lorkhan because he did not know how. If he had, he could have. However, that would require not destroying the Heart (since it apparently has to continue existing for the False Gods it spawns to maintain their power). No destroying the Heart = No finishing Morrowind's main quest.

1: 6 years, correct me if i'm wrong.
2: If it suits your roleplay, go ahead, but according to lore they're different persons.
3: Wait a second... You believe and trust Vivec?! HAHAHAHAHA! *cough* With the right education, the Nerevarine should've been able to connect with the heart, but it would've been of no use, since your former friend with a mental illness is bashing your head to pulp, and wont stop doing it until you free the heart.

The only relevant answer is: if you believe it to be feasible. It is your game.

Hmm, ok. Thanks for that guys

Regarding the last one, using the heart to gain immortality, let's say that the heart was destroyed, Dagoth Ur killed etc. What would it take to rebuild it afterwards? Or is it totally impossible? I know it's impossible in the game, but as a theoretical query...

The Heart wasn't destroyed, because Nirn hasn't melted into astral gloop.

The Enchantments are gone, and it appears to have vanished, perhaps no longer manifesting as big bloody organ.

Well, I'm pretty sure the Heart was that of Lorkans, and is not 'buildable', since it was the literal heart of a god. So I would say no, its impossible to rebuild it.

It is possible to build a new Mantella, which is the important part. Kagrenac made one, Tiber Septim did it as well. You just have to know what it takes to create one.

Didn't Zurin Arctus create the Numidium-Mantella?

Sort of. He served as the source.