Replacing one or both SHL5 super tweeters.
To answer your question ....
1. There have bee three generations of super tweeter on the SHL5. The cut-out hole is the same for all. The connectors were made to be common (in generation two this was by using a flying-lead adaptor). Unless your SHL5s were made after autumn 2008 the surest way is to replace both with the current (and ongoing since 2008) super tweeter. Whatever super tweeter you actually have, anticipating a long production life and then after-care for the SHL5 we made sure that the connectors were common. You can't connect them the wrong way round because one connector is fatter than the other.
2. You should be able to fit this yourself with only basic DIY skills.
3. No soldering. For this very reason of user DIY servicing we do not and never have used solder connections. But you really should use a pair of pliers to hold the receptacle (the end that the s/t's connector blades push into) in one hand and the super tweeter's terminals in the other and slowly wiggle the terminal out of the receptacle. Do not use force and tug them apart of you may rip the wires out from the connectors. Slow, gentle action is the best way.
4. Again, with consideration of ease of servicing the super tweer is fitted from and exclusively accessed by removal of its front plates four screws. You do not need to open the cabinet at all. And yes, we have designed the wire loom that feeds the s/tweeter to be just long enough for you to get access to its terminals. But take care you don't drop the wires inside or you will need to open the cabinet. I'd suggest that you have a helper to hold the wires as you disconnect them or to have some sticky tape cut to size and within reach so that you can temporarily attach the disconnected wires, one by one, to the surface of the baffle as you work.
5. The screw is definitely a PZ type (not a Phillips type) - normally we use the PZ2 for all general screws including the super tweeter screws. Take care not to slip and destroy the diaphragm. Try not to 'mash' the screw heads with a poor-fitting screwdriver or too much force. If you do, then pick out any shards of metal (steel) with tweezers before they are magnetically attracted to a drive unit. If you have broken through the screw's black coating you can always touch it up with a permanent black felt pen so it doesn't catch the light.
Hope this helps.
Alan A. Shaw
Harbeth Audio UK