1. Ideally, at least 2" of thread. They do not need to be fully threaded as long as there is at least 2" of thread. The diagonal holder adjustment needs longer screws now that I know where the mirror needs to be. It does not appear anyone makes these longer than 1.75". I could use a non-socket head screw, but it is too easy to lose grip on the head and drop the screwdriver. Also, a dropped socket head wrench this size will do less damage if it hits the mirror. (Remember the dropped wrench in that Titan II silo.) This may be the solution, depending on the thread length of the >2.0" long screws.
2. The other item, somewhat easier to find is a 1/2" 8-32 hex head screw. Working in the confined space of a 3" telescope tube means that I cannot used a socket head screw. A hex head screw can be held with a small adjustable wrench while tightening down the acorn nut on the outside. Easier solution to #2: put the acorn nut on the inside of the tube.
A ball end hex driver will probably do for holding that 1/2" screw. This allows an angle to the screw head engagement. Looks like a screwdriver. The other option would be a wobble-drive extension in 1/4" to drive an allen head bit. Individual bits are hard to find in a ball-end profile, but common in straight shafts.
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This is 3". they show up to 4". Doesn't indicate thread length. You'll need to call for that info:
https://www.fastenal.com/product/fasteners/sockets/socket-cap-screws/600040?categoryId=600040&level=3&productFamilyId=30635&aggregationSection=true&Dia%2FThread+Size=%238-32&showAll=Dia%2FThread+Size&selectedAggregation=Dia%2FThread+Size