From a very quick skim through:

Laguna just stood there, then, when he had stopped, she turned.

"just stood there" sounds far too colloquial.

Person who live in glass house should not throw stones, person in 3D environment where other person has total control should beware of cheating.”

Is the almost pidgin English-like speech for characterisation? If not Id change it to "people who live in glass houses" etc.

He starred at her puzzled then an invisible wall hit him

Typo, should be stared. It also sounds a bit disjointed, for example "he stared at her, puzzled, and suddenly an invisible wall slammed into him." Although Im not even sure what the hell is going on with invisible walls, again I assume its some sort of technology or something? Or are you just being descriptive, i.e something slammed into him with the force of a wall?

As he arrived Phelps wafted her hand up wards, he hit the ceiling, then collapsed on the floor.

Typo. Should be upwards. Also Im not sure about wafting someones hands? "As he arrived Phelps gestured upwards with her hands sending Slade crashing into the ceiling" reads better to me.

Slowly he struggled to sitting position

Slowly he struggled to a sitting position.

his rear end toasting nicely

Rear end sounds too colloquial. "Posterior" would be too fussy. Id try "behind". You could even get away with backside.

His eyes then noticed the circling fin near the other edge

It feels as though it is lacking a description, its jumping from fact to fact. Try "his eyes quickly noticed the unmistakable shape of a circling fin near the edge of the pool. Slowly it turned"

Laguna placed his body, back down, flat on the table then undid the fasteners.

"Flat on the table and (unclasped, released etc) the fasteners"

his arms were not strong enough to pull him anywhere, or prevent anything.

Lose the comma and add change "or prevent anything" to something like "he was powerless to prevent anything from happening"

nor would the fingers hold onto the towel

Whos fingers?

too departed to join the nappy in the wash.

Again too colloquial. Try "departed, landing alongside the nappy in the laundry basket"

she kissed his bum, both sides. “Now for the powder.” Talc rained onto his groin area and bum

I cant take this line seriously.

Needless to say I hadnt spellchecked it properly before posting, its just as written, but thanks for pointing out the typos.

Person: shes using it to emphasise singularity, in this case him. Its also used in Confucius style quotes, whether really Chinese or not.

Invisible wall:hes in a 3D computer environment, which should be obvious, shes in control. Action therefore need not have any balance with effect. Ergo she hits him with an invisible wall. She doesnt really have to move at all.

Rear end changed to backside, ass or similar didnt fit.

Nappy fasteners are now tape, so unstuck the tape fasteners.

Its jumping from fact to fact on purpose with the fin, its her going faster than he can take things in properly. Youre right that might be a difficult point to balance though.

Fingers, they arent his, they are what shes lumped him with, OK, not saying his is fuzzy, corrected to  nor would the fingers he had hold onto the towel.

I cant take this line seriously.

Never watched a mother change a babys nappy? Yes, this really DOES happen. I know - it happened to me. You probably just dont remember, ask your mum. Really its included to embarrass the character as well as being factual.

For me, bum will do, at least until it gets edited, the only real alternative Ive came up with is bottom. I chose bum because its what an average mum might normally use. Unfortunately, while I know groin area sounds wrong, no other acceptable option comes to mind easily. Loins, what an American might use, is even more colloquial than Groin

What I am trying is depict in narration a nappy advert rather than anything else. Thats difficult to get right.  As with feminine hygene or toilet paper adverts there are some things that will not be mentioned. You wouldnt see an andrex advert with it really cleans the sh*t off your arse well in.

Its not meant to be entirely serious, there is a bit of humour meant to be in there too.

It may be a third person but its as if you are you looking over someones shoulder. Its the same as using camera angles in a film to indicate who the view is from. I understand the issue of me not being a mum. I want to indicate a cosy mummy/baby relationship but from his perspective while remaining a third party.

Really the talc should be rash cream now but its artistic licence, tough.

Really he doesnt want to even consider the issue... How does this sound?

She kissed his rear, both sides. “Now for the powder.” Talc rained onto his nether regions

Ive also changed it from her turning him over to just lifting his legs.

Who has used scrotum in a fictional work recently? And its not only the books level but that of the individual. Ive seen books where some swear constantly and others not at all. It also depends on the individual scene, what the writer is trying for. Dangerous scenes often involve swearing, others might not have any. The problem is no matter what you put it wont work for some people.

I keep the sexual ones to a descriptive minimum, you should know whats going on but the action is glossed over. Like in a lot of other books, if they want to bonk, they do it outside the storyline as much as possible. I dont fancy picking up one of those dodgy sex awards

I got the impression they were shooting out perpendicular to the galaxies axis, and then returning.

When you say give-away, I am guessing you mean cop-out? I certainly didnt expect it until it made itself known!

It certainly is a marmite ending, love it or hate it. Book 3 takes your virtual reality idea on a slight twist

Well, one thing that could be said for the sudden change in direction, is that it kept you on your feet hah. For such a critic to not expect it can only be a good thing

I didnt think the shapechanger made anything obvious, I didnt even suspect until it outright made itself known.

As to the collider around jupiter, you forget one vital point. The reason he chose that is that the nanoforges or whatever will be using it as a resource base. IE, when they turn it on, jupiter wont be there any more. Of course it does cause issues with other planets and orbits etc. But just take it at the level he wrote it for, and go on the size of the damned thing, and that as far as potential doomsday weapons go, its pretty good.

Honestly Serious, Ive discussed various science fiction texts in lectures and seminars with groups of postgraduate English students and well published lecturers who arent anywhere near as pedantic or picky as you. I wonder if you actually get any enjoyment from anything, life must be a constant string of disappointments if you approach it as a hypercritic.