Both are bad options because they passive aggressively try to solve what is ultimately an IRL social issue with some kind of in-game contrivance.Or people accept the Paladin and boot the Nethermancer?Or just leave the Paladin at the local bar and never go back.
There's nothing that says the group has to accept the newcomer.
That's why when I run games we sit down as a group and make characters together. When there is an open spot the new player has to adjust to what they group already has.
Personally, I wouldn't allow the Necromancer. However if there is a necromancer someone wanting to run a paladin would get the veto.
That was a thing with my groups they wanted to kill the group member that didn't work. like guys just kick them out of your adventuring group or like leave real early while they are asleep.
Frankly, that should not happen unless the GM wants it to happen for "learn a lesson" reason...
Else this is a miss from the GM...
When I begin a game as a GM I go with "what do you want to play" with each players... If as a result the group is unbalanced I ask the players that I know are likely to "like" changing class to take the missing role...
And it's the same role play wise... If the group as a whole prefer to play "borderlines Characters" (Meaning mostly Neutral tending on Evil... :p ) and then a new player wants to come in with a LG avatar then two things comes in mind : first discuss with the wannabee LG and tell him/her that playing such a character in this game won't end well... Second if first doesn't convince him/her then let him get in with a tavern encounter that will surely convince him that the players around the table are not going to cope with his LG alignment and surely not take him/her along just because he/she's a player and for that need special commodity from other players... ;)