The corrective lens is necessary or the images will have severe spherical aberration.
You should remove the lens to collimate, though.
I would also put a center mark on the primary so a normal inexpensive sight tube + Cheshire combination tool $40 or a collimation cap ($10-15) can be used to collimate the scope.
Then, look at the stars (if you can achieve focus).
If the star images are round but just a bit "blobby", try then installing the lens.
If the star images then go to long ellipses, you will have to observe without the lens or figure out how to tip the lens to flat so the star images are not flared.
I would guess you'll have to observe without it, but be aware this means the focal length of the scope will be about half as long.
As for putting a mark on the primary, it's easy:
Remove the mirror and its cell from the tube by unscrewing the screws on the side of the tube that hold the cell in place on the end of the tube.
Be sure first to mark the tube and the cell next to each other so you can put it back on in exactly the same attitude because the holes may not be perfectly spaced around the tube.
Cut a piece of heavy paper out in a circle the same diameter as the mirror,
Fold in half and then in half again so it looks like 1/4 of a pie.
Nip off just the tip with scissors and then open the paper and make it flat. There will be a small hole in the center.
Place it on top of the mirror and push a Sharpie pen down through the hole to make a dot on the mirror.
Then, use a pair of tweezers to place a stick-on paper reinforcement ring (the type to keep holes from tearing out in a 3 ring binder) on the mirror with the center of the hole being the dot on the mirror.
This will all be in the shadow of the secondary mirror, so no worries. Press it down with the eraser end of a pencil to make it stick.
Replace the mirror in the tube and reattach the screws. Ta da!
Now, for collimation. At a minimum, a collimation cap. For a few dollars more, a combination sight tube + Cheshire like the Celestron Collimation eyepiece.
(or Orion, or Zhumell, or any other brand). After collimation, follow the star image inspection procedure I mention above--first without the lens, then with the lens.
If the lens really messes things up, a 2X Barlow lens may suffice to replace the lens in the focuser. You'd use every eyepiece in the Barlow. At least the lens will be correctly oriented.