How many trailing blocks is needed to guarantee that previous blocks are not affected any fork.(immutable)

Recently updated block explorer site looks good.
I have some questions.

How many trailing blocks of a block is needed
to guarantee that previous blocks will not be affected any forks.
The all blocks including head block on the block explorer are immutable?

Is there any update policy(such as update interval) on the block explorer site?

If a mining server makes(mints or receives from peers) a block
which is different from the block explorer under the explorer’s top height.
I think it is a power and time wasting work.

I think like below. Am I right?

case	My head		Block explorer head		Same blocks		Future?
1		#100		#100					#1 ~ #90		:( My Blocks from #91 will be disappear by fork 
2   	#101		#100					#1 ~ #100		:) Maybe I minted a new block.
3   	#90			#100					#1 ~ #80		:( I have no time to mine, I'm trying to catch-up the latest block.

Does the official nodes(tcp://node-*** keep the latest block?

Is it possible to receive or solve the sha256 quiz before getting latest block?

The block explorer shows a version of a blockchain from a single node. We plan to improve the confidence by adding multiple nodes behind the explorer, comparing data from each other.

In the current version of Creditcoin, with fork resolution issues, the last 30 to 50 blocks are likely unstable (or 30 to 50 minutes). In other words, you would need 50+ block confirmation to get confident with the data.

Our next release, Creditcoin 1.7, will tighten the network. We expect forks to be within 10 to 15 blocks. We will monitor the network to see how many diverse tails are on different nodes after the release to confirm.

That is exactly what I want to know.
Thanks for the your detail explainations.

And Yeseterday,
I found the official node(tcp:// was the only single peer connected to my server.

But my server’s stale chains grew up for an hour.
And finally failed to catchup latest block state.

The Official node guided wrong way.
I can’t trust the official nodes.

Why did this happen?

You cannot. You have to know the latest block to solve the SHA256 quiz for the block.

No node has no special authority within the network, which means any node can follow a fork that gets abandoned in the future.