I’d ask your neighbour if you can fix a plate to their wall – will only need 2 expanding bolts. Unless you dress a flashing into a (routed out) mortar joint you’ll never get a watertight joint of the wall on the left to the roof (same on right).
I’d live with this as a mastic joint to dressed up roofing felt would be sufficient for bike storage, but will need re-doing every couple of years. Its outdoor storage, not a living room!
Perhaps just compacted gravel on the floor which will be far easier to lay around the manhole than a hard floor, and will drain any water/drips and can be raked over to hide mud off bikes.
Re comment above, I don’t see that brackets will be stronger than a ply deck screwed directly into joists for security.
Also slope the roof to the front, if necessary put a short length of gutter over the door running to a downpipe into a shingle soakaway – its a small area so won’t collect much water. If it slopes back, all the leaves and shizzle will collect there and block the outlet which will be a nightmare to access and will safe all the faffing trying to connect into the downpile, then trying to construct some sort of gutter which will also seal to the rear wall – nightmare!
Perhaps hang the bikes from the roof on hooks as you can then duck under them, lift down the one you want and duck out?