Receiver can pay fees too when sender doesn't want to increase fee or doesn't have control over the keys using CPFP but sender has to pay the minimum fee possible to broadcast the TX.
I doubt any merchant bother do this, since they release the goods/services only after the TX got confirmed. I expect only few individual user who're waiting for withdraw or payment who may create CPFP TX manually.
AFAIK, Electrum always take fee from the funds left not from the specified amount so if you want to send any amount including fee then subtract the fee that you can see while sending.
Yes, thats the way I could do it. But thats uncomfortable.
I don't want to manually calculate that. Electrum should do that for me
Your use case is uncommon, so i doubt Electrum will support such feature. Although you can use tool such as
https://bitcoinops.org/en/tools/calc-size/ to get TX size and
https://mempool.space/ to get ideal fee rate, in order to make it less uncomfortable.