When using a VPA (Variable-Pitch Connector) to connect roof rafters to a top plate, how and where do you install hurricane ties

In order:

a) You can do it! You didn't mention anything about trying to move the existing structure, so this is based on my understanding that you are simply adding the tie back in to prevent further spreading. Personally I would use something more like a 2x8 or 2x10; a 20 ft. span is a long way for a 2x4, and you'll likely see some sag. I would put 5 nails into each rafter, in a rough square pattern with one in the center. But, you'll want to offset the nails a bit from each other so that they aren't going through the same grain (which can make it easier for a crack to split the wood where the nails are). You can cut the ends at the same angle as the rafters to get a longer surface to nail through.

b) If you do end up hiring someone else, a handyman could certainly handle it.

c) From your pictures, one tie every other is fine: the plates of the walls aren't going to move with a tie every 4 feet, and additional ones aren't going to do much.

The purpose of the metal tie specified in R602.6.1 is to ensure lateral distribution of the vertical load born by the top plate. Non-bearing walls have no vertical load on the top plate and therefore no metal tie is required in this chapter. You would still need to meet the requirements to protect piping, wiring, or equipment (.e.g, M1308.2 and P2603.2.1).

I haven't found a good discussion of a centered hole in a top plate. I would argue that IRC Chapter 6 requires only one metal tie under any circumstance and if the hole is centered then it doesn't matter which side you install it on. But again, if we are talking 2x4 framing then pipe/wire protection may drive a requirement for metal ties on both sides of an interior wall.

Regarding double top-plates see IRC Interpretation No. 15-03

Q1: Where a wood stud wall is capped with a double top plate, is the 1.5-inch metal tie as described required on both of the top plates?

A1: No. Where a double top plate is used, a single 1.5-inch metal tie is required on the uppermost plate only. No tie is required at the notched lower plate. This is illustrated in Figure R602.3(2) and Figure R602.3.1.

Simpson has a good reference for Intallation of Utilities in Wood Frame Construction. Combines structural and nail protection requirements for residential and commercial codes. If you are framing with 2x4's it is likely that the pipe protection requirements will push you to use PSPN516 (load bearing) or PSPN58 (non-load bearing) rather than one of the RPS repair straps.