Genuinely, thank you for the well thought out response
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While that is cool and probably works here are some things to consider.
Bot lane
Pressure lost: Shes not there so it's very unlikely that the enemy will not threaten or kill bot.
This is true, and I did think about it. It would really be best if the bot lane player is aware of the strategy as well. They will need to play more passively while Senna is gone. The good news is, THEY will get more experience advantage over the enemy lane while Senna is gone. This could actually be an advantage, especially if the bot laner plays a mage, which also has good synergy with Senna being a physical damage dealer.
EXP: She will lose very valuable EXP about 2 waves worth for being gone for 1 minute.
Likewise, I agree this is also an issue. The purpose here is to get an aggressive start. Roaming with Ivern to collect wraiths would lead to a spike in AD and place her closer to her first 20 wraith milestone when she returns. Also, when they end up top side, they may have an opportunity to gank top or mid together, leading to more wraiths and kill gold.
Other Wraiths: Senna gets Wraiths for being in lane as well so she is losing those for the gain of others.
This might take place during the first cannon wave. Senna will likely have got 2 wraiths from minions and as many as she can from harassing. She might miss out on 3, maybe 4 wraiths at most from minions. If she wouldn't have got any less than 4 wraiths from harassing enemy champion in that time, she will be coming out ahead in this area.
Support item: Not stacking/poking with your support item will hurt a lot.
fair ..
Jungle side
Invades: Ivern might get invaded. Should that happen he'd be level 1. Most Junglers ward early so they will know that something is up if they see a level 1 Ivern at 2:30-3 minutes.
In my experience, Ivern is very healthy after the teleport. Having Senna with him would likely turn an invasion into a kill. As for the grove placement, .. Ask top and / or mid to ward top side jungle entrances around 1:20 and you'll know if they're invading. More people should really be doing this anyways. Knowing they're in your jungle means your lane can collapse if they have priority, and they get a kill or at least waste the enemy jungler's time, putting them behind.
Also, Ivern is level 2 after smiting the first buff.
Lane Pressure: Counter ganking or scuttle is not an option for Ivern and thus the 9 wraiths surrender his lead to the jungler.
This is just wrong, .. Ivern has the easiest time of taking scuttle crab and invading when he has smite.
The first scuttle crab spawns along the route and he will have smite off cooldown with no other camps to smite.
An Alternate route that mitgates the cost a bit is.
As for all of this, it might be a better compromise, but I feel like there would be a lot of time wasted. I'm looking at pulling Senna out of bot lane for a quick power spike and applying potential pressure on another lane, then she can go back and do whatever she wants.
A single clear path allows Ivern's whole jungle to be completely cleared at once. He will be free to gank any lane, even returning to bot lane WITH Senna.