Is it safe to store mnemonic seed in the email if I change random 2 words? If I store 12 word seed in the email like gmail and my phone as well but change 3rd and 7th words in the seed for example with other bip-39 words and remember it, would that be safe? I can't think of a way for someone to be able to efficiently brute-force it.
It would be trivially brute forceable.
There are 2048 BIP 39 words. If you change two words AND the position is known, it's simply a matter of computing all possible combinations of what those two words can be replaced with - 2048*2048. That's only 4194304 - A smartwatch could do that in a few minutes without breaking a sweat. It would take you longer to write a script to run through all of those than it would take for the script to run.
If the positions of the changed words aren't known, it is slightly harder, but not extensively so - the first words can be placed in any one of 12 slots, and the second in any one of 11 - that gives you 2048 * 2048 * 12 * 11, or 553648128 possible combinations - slightly better - the attacker might now have time to make some coffee while he steals your coins.
If you genuinely want to security your mnemonic, you should consider one or more of:

Encrypt it with AES, store the encrypted version and the password in two separate places
Split it into M parts using Shamir Secret Key sharing, store each part in a different, secure location
Write it down and store it in a secure vault or deposit box.

You can combine the above to protect against various threat models.