First of all, you need to know how to differentiate between transaction size and weight. Sat/vbyte is using weight while sat/byte is using size. Blockchain.com wallet is using legacy addresses, that makes the weight of the transaction you are making to be higher than using nested segwit and native segwit addresses. Do you know you can safe 42% fee or higher while using native segwit addresses, and also you can still safe much fee even if using nested segwit addresses. That is why I will recommend you not to use blockchain.com wallet again as it did not support segwit. Be using electrum, it generates native segwit by default, you will be able to pay much lesser fee. You can read the link below to understand more about this.
Minimizing bitcoin transaction fee1. As you know Blockchain.Com has no RBF option. So those two transactions have n't RBF flag.
In this situation can i use double spend with electrum?
Double spending in this case means you will be able to use RBF to pump the fee, so that the transaction can be confirmed earlier, although another txid will be generated, but if high fee is paid, the transaction will be confirmed earlier. But, both your transactions are not supporting RBF, which means you can not double spend it.
2. How can i create a transaction in offline mode and sign and save it?
I tried it.
This is another question entirely, and does not related to the reason why your transaction is stuck, but this is the procedure (for safety reasons):
On computer:
https://electrum.readthedocs.io/en/latest/coldstorage.htmlOn mobile:
https://bitcointalksearch.org/topic/m.541493633. If i double spend first transaction what happen to second unconfirmed one? (I am a bit confused about this)
If you pump the fee for the first transaction, if sufficient fee is used, the transaction will be confirmed in time, and this will increase the chance of the second transaction to get confirmed earlier as it has no unconfirmed parent transaction again. But 99 sat/vbyte while sending from legacy address to nested segwit address is not enough fee for now as the mempool is congested ever than before for now, it may still take long for the child (second) transaction to be confirmed. But if RBF is supported for the child transaction as well, you can decide to pump the fee also, But, both transactions do not support RBF, and you can not use it. Better try to wait until the mempool is very well less congested. If you pump the fee for the first transaction, if sufficient fee is used, the transaction will be confirmed in time, and this will increase the chance of the second transaction to get confirmed earlier as it has no unconfirmed parent transaction again.
This is not correct. If you double spend the unconfirmed parent transaction or pump its fee with RBF, then this means the parent transaction is being replaced with a brand new transaction. Doing so will invalidate the unconfirmed child transaction. Even if your new parent transaction sends the same amount of bitcoin to the same address(es) as used by the unconfirmed child transaction, your new parent transaction will have a different TXID from the one it is replacing (since something else will be different about the transaction - at a minimum, how much is being paid as a fee). Since transactions rely on the TXID of previous transactions to identify which inputs to use, by changing the TXID of your parent transaction, the inputs the child transaction uses will no longer exist and so the child transaction will be invalid. You would have to recreate your child transaction using the new outputs from your new parent transaction.
The only way to pump the fee of the parent without invalidating the child would be to use CPFP on a different output from the parent, which will speed it up without replacing it with a different transaction.
4. Should i double spend two unconfirmed transactions with two temp wallets in electrum or double send of first one is enough?
Both the transaction do not support replace-by-fee (RBF), so you can not pump the fee, which means you can not double spend any of the transaction.
I will advice you not to use blockchain.com wallet again, it is not supporting segwit transactions, and also not support RBF. If you need a wallet that support both, use electrum. Electrum can help you in the case like this, while you can pump the fee by double spending the bitcoin with higher fee, and yet using segwit for lower fee.