It's possible, but there is a couple of DM approvals required to kick this off
Let's start at the beginning, you've got a familiar that wants to be 'on' your character as a means of providing 'safer' Help action economy.
Mounting a player character (DM approval)
Generally, two characters can not end their turn in the same space. The question of mounting another creature then comes into play. This is the first step in a DM determining whether or not something is feasible. While RAW it's possible, a DM still needs to determine if a spider can actually mount a humanoid. It's not unreasonable to say that a spider does not mount a humanoid and that the proposed combo isn't viable.
But let's say they do.
Squishy Familiars and Action timing
First off, find familiar doesn't RAW allow the caster of the spell to control the familiar on their turn. In fact, the familiar needs it's own initiative (PHB, 240):
Your familiar acts independently of you, but it always obeys your commands. In combat, it rolls its own initiative and acts on its own turn.
Again, the DM will need to houserule to allow the timing of this to always work. Of course, if giving it's own initiative and they're after the Rogue, they can always hold their action to Help as the Rogue closes in.
But let's say this all works out in terms of initiative order. At this point, you still have a very squishy little spider. It's easily killed via AOE attacks or most direct attacks due to it's low AC and HP.
There's still the option of not using Help for advantage and Sneak Attack, but just being within 5' as you've noted. Then the timing is less important except for it being on you at the start of combat.
And there remains a cost to bring it back the next combat. If the caster isn't a ritual caster (like the proposed Arcane Trickster), they're going to need to burn a spell slot and the required materials. It's not a bad trade-off for the increased damage output with Sneak Attack, but it is a cost that's higher than a caster with Ritual Casting.
Being an enemy (DM Approval)
If not using the Help action, then all the creature is is a spider. It's not involved in the combat or the encounter in anyway other than simply being present. If it's not actively doing something, it's a bit of a stretch that someone would consider it an enemy.
What's good for the goose...
Finally, we've got a player using a semi-reasonable tactic. The tactic is more commonly used with the owl's flyby and trusting that a creature won't target it after, but this is very similar to that. It's a big part of having a familiar and isn't necessarily bad.
But if it's always working, I'd fully expect the DM to set up monsters who also use this. The biggest thing that I as a DM always try and work with is allowing players freedom to come up with cool stuff, but if it's borderline and used too often, then i'll have a talk with them.
Owl Fly-By and my experience
When I had my wizard (before he died :( ) I used the fly-by owl to grant Help. I did it more often for my friends then for myself, but I wasn't the class that would gain a real benefit like a rogue (I did generally give the Help to the Rogue.) The DM was skeptical, but understood the mechanic. And at times they directly targeted the owl, but at other times did not. It was really based on the monsters, their intelligence, and the threat levels. But a monster realizing that this thing is making another creature really hurt them, they're gonna get rid of that thing.