Good deck of questions but I don't think they matter. I don't think those are answerable questions for observability, be it OpenTelemetry or other proprietary systems.
You can go read the leading observability companies web pages and they'll have a 4 page writeup on custom instrumentation. That's not much, just covers very elementary basics! It's not like OTel is behind. The answer just tends heavily towards "it depends."
Once you have experience - OTel or other - you can work through these things that might confound a neophyte.
The problem is they keep making the OTel tooling worse for working through these things, because the people writing the OTel tooling broadly aren't the people actually trying to monitor things. Even before OTel, plain Prometheus client libraries suffered from this.
You can go read the leading observability companies web pages and they'll have a 4 page writeup on custom instrumentation. That's not much, just covers very elementary basics! It's not like OTel is behind. The answer just tends heavily towards "it depends."
Once you have experience - OTel or other - you can work through these things that might confound a neophyte.