1.a. its more so that when a new block is added the previous/older block(s) are safer to trust as stronger. EG a transaction in a block 100 blocks ago (100 confirms) is safer than a transaction with only 1 confirm

1.b. because a block contains the hash of previous blocks then to have a valid block does require clear links that do go all the way back to the genesis block. for instance you cannot just copy ethereums block history or litecoins block history to be a bitcoin attack chain. because although their blockheights(number of blocks) are more. the genesis wont match

2. https://en.bitcoin.it/wiki/Block_hashing_algorithm

3. yes each tx has its own hash which is called a TXID (transaction identifier)

4. he can only ultimately alter which transactions were in a block. thats the easy part.. catching up with the network after is the harder part
say blockheight was 570000 and someone wanted to edit out a tx in 569999. they would do that but then they would need to PoW mine the hash for their 569999 and then add that hash to their 5700000 then PoW hash that block.
the rest of the network would probably be on 570002 at this point because while he went back everyone else continued forward
so the person editing would need to catch up and race against the network hoping to get ahead so the network would accept the alteration.. which at 15% has little chance of happening

5. save as last point but much more chance.

6. in simplified terms the block header has a nonce. ASIC create a hash that is the block header and where the nonce changes. this makes the hash change. they keep changing the nonce until they find a hash that meets the difficulty requirements

7. imagine it like 15 different groups(pools) racing each other to get a block hash result that meet the rules. only one can win per block.

8. continuing from point 7. the other pools dont get anything. its like a 100m race the first one to cross the line wins.. as long as they are not found cheating

6. the nonce. is just a space to put in a number.
imagine you had some text :
'blahblahblahdeblah'
the text and a nonce would be like
blahblahblahdeblah000000
if you hashed it you would get
161F37591904929E6EBFDD9BA4DFB22C092204DE58D552E46FF9F771AAF186D4
so changing the nonce changes the result
blahblahblahdeblah000000=161F37591904929E6EBFDD9BA4DFB22C092204DE58D552E46FF9F771AAF186D4
blahblahblahdeblah000001=86E8E440341479C85888F906EE103579BE4EF34582FDECB546BBF43AC3C3A904
blahblahblahdeblah000002=887C68F2C56015B3CBFE4D3E76531AA2A1B4855CE9394940F783B0BE01BF1ABC
blahblahblahdeblah000003=E110996EBDBFFD6FB63D78FEABE2AF3F89577CE031A9764C6D6C4C98F3711794
blahblahblahdeblah000004=576889D3D11E55ACA59B4176B2119ED8B7DD8B571C0622AEFB0B4564F1D2D71D

7. if a few pools crate blocks within seconds of each other there is a chance that different users would get different blocks. depending on how they are connected to each other.(a small rar chance of this happening)

ALL the pools then restart working on the next block. and again whoever is fastest wins.
and this time the gap between first and second place might be wider so now the winner is a clear winner which then makes all users choose to trust the fastest clear winner. and so will hold onto the blocks that are linked to the winner chain of previous linked hashes and ignore the second place

people in second place dont win anything. they just have to retry and hope at some point they gt to first place.

due to issues where users might (rarely) hold a different block due to the race competition.. block rewards are not spendable instantly.. but instead untill 100 other blocks are stacked/linked ontop because the chances of people editing blocks and then catching up (point 4) and also the races swapping latest blocks. are not going to be a concern to a block 100 blocks deep(100 confirms) as the odds of negative impact resulting in changing something 100 blocks ago is soo small. its then deemed safe to let the winner finally spend their rewards of the block they won 100 blocks ago