Hi Klem,
first off I wonder why you didn't mention pin16 (VDD) of the HEF4027.
Together with pin8 it makes the power supply for that IC.
The power supply is the first item in the schematic on the left labelled 'IC1 P', meaning IC1 Power.
Pin15 is not connected on purpose since it's logic level is opposite of pin14's level, meaning if pin14 is high pin15 is low and vice versa.
By the number of experiments you have done so far I guess you'd be better off making an underwater floodlight of acrylic glass with the battery (SLA 6V/5.7Ah) and everything else encapsuled to make it 100% waterproof and deep sea going.
Acrylic glass is easy to handle and can be glued using the kind of glue camper manufacturers do. Basically it is acetone with a an additive of arsenic.
Using acrylic with 10mm thickness you will be able to dive to 100m without breaking it (provided the enclosure is not too big, or slightly arc the material applying heat) To make it safe you might equalize the inside pressure close to ambient pressure by inflating a baloon inside. (just the opposite of aircraft pressurization)
Further you can drill 'threaded sack holes' into it to mount any fixture inside.
In the meanwhile I have thought about the SMD-version, since the wired version is considerably high requiring lots of space.
The newly designed SMD-version has dimensions of 1.35X0.7625inches, using power MosFets type ZXMN05B14 (Zetex semiconductors), and available at Mouser for 31Cent each.
Even the IRLZ34 as D2Pak is 2,000% overkill for the load you intend to drive.
The mentioned transistor can drive loads up to 3.2A (still 320% overkill)
and is available with an SOT23 package.
Kind regards
Boncuk