This is a good point and completely true, but I also think they made a packet out of the comics. If you consider that when DC Thompson published the Beano, they had to create the whole thing, pay the usual bullpen of writers, artists, editors etc, physically produce the comic and print it, they did all that and sold it (using 1967 as an example) for 3d. T&P were not doing any of that, just buying & shipping the finished mag (at wholesale prices in the case of Marvel, and as distressed, second hand inventory in the case of DC), yet they were selling it at FOUR times the price that DC Thomson were getting for comics created from scratch and printed. And if it didn't sell the first time, they just halved the price and sent it round again. I think the profit margins were astronomical.
I agree the profit margins on the porn must have been significantly higher, but I imagine they sold far less copies and the amounts seized were mind-blowing. Don't forget that T&P completely got out of the paperback business, lucrative as it was, in 1952, because of the amount of police seizures, which surely indicates that whatever the margins, the problem is that you have to actually sell it to make any money on it.
To your point, T&P in fact sold a wide range of goods through newsagents, including comics, magazines, books, records (not just the ex-juke box records, but original LP's). They sold ladies tights & stockings (and I'm not sure what else in that arena). And children's toys. I think that part of it was massive. There was a whole toy department at Thurmaston (like Santa's workshop, except with massive amounts pornography stored next door).
I could never quite figure out how (or more importantly why), they maintained this massive operation of sales managers and regional warehouses when it was perfectly possible to distribute comics and magazines to every newsagent in the land via the national distributors, but I think this is the reason. They operated their own stands & spinner racks at the newsagents, the sales managers themselves replenished the racks so they maintained tight stock control, saw what was selling and what was not, and, to your point, kept the high margin items well to the fore. One envisions it as T&P supplying goods for sale to the newsagents, but actually I think it was more like T&P used the newsagents to distribute their goods and pretty much ran a business inside each of the newsagents' businesses.
I think your point is also very true of the US distribution network. I think the reason that a lot of the returns came back in the same condition they went out is that they never made it onto the news stands. If you were a news vendor with a tiny kiosk, there's no way you'd give rack space to comics on which you made a penny when you could put out Life (at 40c) or even Playboy (at 75c). You took the stack of comics along with the newspapers and magazines because you had to take what the wholesaler gave you, but it was all SOR and it was up to you to decide what you were going to give up precious real estate to.