Ethereum 2.0 consists of multiple phases. Phase 0 is the beacon-chain (with proof-of-stake). Phase 1 is sharding. The beacon chain is expected 2020-2021. Shards are targeted 2021-2022 (although there is slight parallel development with the beacon chain).
There can be subsequent phases but they are further out so can change. For example, Phase 2 is about state execution and cross-shard transactions but...
More importantly, "Ethereum 1.0" and "Ethereum 2.0" will be merged before Phase 2.
See https://blog.ethereum.org/2020/09/01/eth2-quick-update-no-15/ for the eth1+eth2 (Phase 1.5 aka The Merge) and its current end-to-end demo.
The questions in the OP are all about the subsequent phases of Ethereum 2.0 and are not applicable at this time. The beacon chain does not have smart contracts. This means that for the foreseeable future, smart contract developers do not need to worry about anything about eth2. It is too early for any detailed answers about eth2 smart contracts, gas costs, oracles, accounts, transactions...
Giving each phase a name would have helped reduce the confusion. Related Casper, Serenity, Constantinople, Ethereum 2.0
There is no longer an Ethereum 2.0 if Vitalik can help it. What we
call Ethereum 2.0 is simply the Serenity release and that's what he'd
like it to be called though I'm sure Eth 2.0 will stick.