Transactions can be up to 100kb in size (to be valid according to the network rules).
It's not. Transactions are valid as long as they can fit inside a single block.
but by saying that, all I meant was the same thing again that after we choose how much should be the min fee/byte and max fee/byte or a slab (as said before), how will those low-fee transactions ever confirm?
When the miner choose to mine them; or if all the transactions that the miner see if about that fee/byte.
Can't this be considered as a spam attack by miners' end too in order to break the rules by making transactions wait longer to make mempool congested in order to let others raise fee just for the purpose of profits?
Yeah. It is.
What if (a) miner/s never decide to take those low-fee (even 100 sats/byte) transactions in their blocks?
Then they don't confirm.
Everything (POWER) is in the hands of miners and if they choose not to include low-fee transactions and only raise the fee ahead, will there be anything that might confirm them (even in future)? Or such transactions need to be repeated with higher fee (possibly double-spends) to get through?
No. If no one wants to confirm your transaction, it would never be included in the blockchain.