No.

PHB, Page 163, Multiclassing

With this rule, you have the option of gaining a level in a new class whenever you advance in level, instead of gaining a level in your current class.

This wording seems clear to me that you must choose either a different class, or gain a level in your current class. In the case of a cleric, this implies that you cannot start out at level 1 again (e.g. Cleric 1 / Cleric 1), as Cleric is not a new class.

Addressing the specific concerns of Cleric (Life) being the same class as Cleric (Storm), page 45 of the PHB states there are twelve classes, and then lists them in a table. In this table, Cleric is found only once, and there is no special mention of subclasses being different classes entirely.

This was officially clarified in the June 2016 Sage Advice:

Can I multiclass into the same class to pick more than one subclass?

Multiclassing is designed solely for taking different classes. For example, you can be a fighter/rogue, but not a rogue/rogue.

That said, I would talk to your DM. It may be reasonable, for example, to mix and match some of the domain powers at their discretion.

Yes.

If you can dash as a bonus action you can move, dash, and then dash again, moving thrice your speed in a round.

There is no stipulation on Dash that you can't use it as an action (if you have the ability to do so) if you've taken it as your bonus action (like there is with casting a spell as a bonus action).