Core dev, Lior Yaffe, interview with Labo Blockchain from Japan https://www.blockchain-labo.jp/interview/nxt_ardor and direct you tube link on Blockchain-Labo Youtube https://www.youtube.com/watch?v=DNz9GJb2wwI

In order to receive Ignis, only NXT balance matters. Or you can buy JLRDA (placeholder of future Ignis) right at your client, by clicking "JLRDA token sale" link in top of the client. Don't buy anything else sounding like Ignis, except through this link.

All is explained in the link I provided. But, anyway:
1) ARDR snapshot and distribution already happened last year. ARDR asset is freely traded now. Another ARDR one time snapshot will happen around 25 December. After this snapshot ARDR asset will be locked for trading in Nxt blockchain. ARDR asset amount you hold in your account will appear in Ardor platform genesis (01.01.2018).
2) See point 1. for ARDR asset snapshot. For Ignis, NXT balance will be snapshotted, on 1 NXT:0.5 IGNIS basis. This will happen also on around 25 December. Ignis will appear in your account during Ardor genesis, same as with ARDR.
3) Genesis block include all snapshots as explained. The same NXT passphrase will open the same account id in Ardor platform. So, keep your passphrase safe, it is all you need in order to control your account.

Mistertango and btc-exchange.com are run by separate companies.

Yes, that's true, snapshot will be for both, ARDR and NXT.

https://bitcoinexchangeguide.com/jelurida/ Here is a good explanation.

Yes, the safest place for your funds is a wallet you control. No one exchange confirmed help with Ignis distribution, yet.

It will be one time snapshot, not like with ARDR, which was 3 month long snapshot. Check roadmap: https://www.jelurida.com/ardor-release-timeline

Check this video: https://www.youtube.com/watch?v=YGZq8R5ljMc
Tutorials: https://www.nxter.org/nxt-tutorials/

First of all, for NXT you'll get Ignis, not ARDR. Here is well explained, how ARDR and IGNIS tokens will be distributed. I would suggest you to install official Nxt client from www.nxt.org, install with light client option (no blockchain download), create new account (keep the passphrase in safe place!!!!),  and send your all funds to newly created account. Mynxt wallet is 3rd party wallet, so better keep your funds in wallet you control.

And forging vs bundling: https://www.jelurida.com/forging-bundling

Ardor Release Timeline

Oct 28 - Nov 04
5th and last ICO round, 100 M JLRDA plus all unsold JLRDA from the 4th round and minus marketing expenses are put for sale at 1.45 NXT each as announced
Any remaining unsold JLRDA tokens will be kept by Jelurida until the Ardor snapshot and the resulting IGNIS tokens will be used to fund future development and promotion
Nov 04
Ardor testnet 2.0.4e release
Starting from a new genesis block and deleting the current testnet
Adding major new features: Asset Control and Composite Phasing
Nov 24
Final Ardor testnet release
Source code is released for review under temporary license, no cloning allowed
Node JS module and Mobile App operational
Only security and critical bug fixes after this date
No further API changes
Four years anniversary for Nxt celebration
NXT Block Height 1630000 - Approximately Dec 25
Ardor genesis snapshot recorded at block 1630000
Transactions with ARDR, Bitswift, and Janus assets disabled on the Nxt blockchain
Dec 27
Ardor generally available software release
Source code released under the JPL
Forging not yet possible to give time for users to setup nodes, and perform final verification of snapshot balances
Jan 01, 2018, 00:00:00
Ardor genesis block timestamp reached
Forging starts automatically
Ardor blockchain becomes fully operational
2018
Continue development according to roadmap
Nxt roadmap
Dec 24-25
Hardfork, freeze trading of ARDR asset and other assets included in the Ardor Genesis Snapshot
Q1 2018
Develop Nxt as a simple and secure crowd funding solution for new ICOs
Position the Nxt Blockchain Creator Kit as a viable solution for new blockchain projects
Add permission layer and peer communication encryption for private blockchain solutions
Q2 2018
Merge features added to Ardor which do not rely on the parent/child chain architecture

Taken from https://www.jelurida.com/ardor-release-timeline

Proof-of-stake.

Most active place is Nxt slack, invitation is at https://nxtchat.herokuapp.com/

For the news: www.jelurida.com

Official timeline for the Ardor release will be published by the end of next week (October 14th). Testing on testnet is only for finding bugs, but latest version works without big flaws. Next version will introduce new features, as core dev said.