Player wanting to switch character for a short time mid-arc (spoiler for my party)

SPOILERS FOR ARKAS, HODY, STAKOR, FOZ. DON'T READ ANY MORE.

I'm running a Pf2e-ported, heavily homebrewed, Lost Mines of Phandelver - Dragon of Icespire Peak crossover. Essentially, it's mostly been focused on the main villain of this arc (which has taken up most of Tier 1) the Black Spider. We've been playing for about 5 months. This player is playing an Anger Phantom summoner - his phantom is a dead paladin he found underneath the remains of a ruined town.

I've woven their backstories into the plot - Wave Echo Cave, the location where the Black Spider is operating from, was believed to be invaded and sundered by orcs - but in actual fact, it was from a brother Order to my summoner's eidolon's. This would be a surprise to him, and start him on the path of learning more about who his phantom was in life.

The only thing is, he wants to play a Fetchlin Rogue for a time, because his summoner character just got a letter ordering him to come to the court of the baron he serves. He says he wants to have his summoner go off and do that as a series of 'cutaways', while his rogue finishes out the current arc. He said he wants to do this purely because he has a 'character he wants to try'. 

In my campaign, the Black Spider is trying to merge the Shadowfell with the Material Plane using a magical forge - the Forge of Spells - to bring about eternal night (he's a drow), so it would fit nicely, but I'm worried about a) introducing a new, temporary character at this point, and how well that would synergise with the party, b) his current character missing out on development and the narrative satisfaction of finishing out the arc, and c) the precedent it sets for switching out characters just to 'try'. I'm hesitant to hit him w/ the flat NO though, because I do want him to enjoy playing and it would make narrative sense. 

Even after this arc completes though, I'm not really sure if I want him to just be switching out characters - or 'subbing in' as he put it - just because he wants to try something out. I hoped you fine folks could give me some advice! Would you allow this? If not, what would your solution be (flat no? compromise?)