Lap recording in multisport mode

Therefore I propose instant press of lap key to activate lap change and holding press of lap key to activate transitioning.

The functionality you want can be achieved in a different way. Do not start a dedicated multisport activity. Just select the sport for the first leg (e.g. running). While running you can trigger as many laps as you want. When you're done running and want to transition to the next sport, just select another sport without stopping the first sport. The way to do this is device-specific, for example on my FR735XT you need to hold the down button and then select whatever sport you want. This way the watch will record the activity as multisport AND you can trigger laps without triggering transition.

I don't know how it'll work on the F5, but I suspect it will be similar.

Interesting bypass (even though it doesn't work on my Fenix 3)

Hmmm, I never had a Fenix 3 in my hands, but other garmin multisport devices have a way to manually switch sports one way or another, so I suspect the F3 has it, too, only by a different method.

The first one is that when you are in a race i.e. triathlon you cannot search for the next sport in the heat of the race but you need a predefined multisport format.

True, but with a little practice and preparation this can be done with minimal effort.

The second one is that you don't record transition times this way.

This can be bypassed by selecting OTHER for transition or even creating a custom activity profile for transition.

I, too, cannot find a way to switch sports with the activity started. I also agree with the OP that the workaround is very cumbersome and mostly undoable in many races. The long press - short press solution would be great. I don't do triathlon but I race adventure/survival and it would be great to keep the multisport format for the various legs of the race (run, bike, boat, spaceship) and still be able to hit laps at aid stations or event/discipline stations to measure segments of the race.

This lack of lapping and flexibility when changing sports in multi sport mode is one of my biggest complaint on the F3HR. And it is one of the things Suunto nailed with the Spartan Ultra already at launch. So I definitely would like to see some improvement on this area when going to F5 series.