I don't get your point about Samarkand. You initially suggested that Russia would want to drive south for that trade power. Persia is more difficult for them to take than Samarkand. If the goal is to simply encourage Russia to go after trade income, why is it helpful to make that target more difficult? I'm not following your logic here.
And I'd beg to differ about those North African provinces being useless to the Ottomans without trade. In my current Ottoman game, with autonomy across the Tunis node at 0, the node itself produces 5.46 ducats per month. Frankly? That's nothing. Tunis produces 1.55. Sousse 1.57. Sfax 1.2. At least five other provinces produce 0.9 or greater (and of those, only Fezzan produces less than 1 per month). I get more money out of the production and tax of those provinces than I would even with 100% trade power.
Besides, I don't have enough traders as it is. I've got 8 traders active. I'm pushing from Ceylon for the triggered modifier. I'm pushing from Alexandria, Crimea, and Aleppo to feed Constantinople. I'm pushing from Astrakhan to starve Russia. I'm pushing from the Gulf of Aden to starve the Europeans. I'm collecting in Zanzibar (soon to move that merchant to Cape) and the Ivory Coast to continue depriving money from the Europeans. And that's all I'm able to do (besides collecting in Constantinople without a merchant) because I don't have any other merchants available. And if I did those most certainly wouldn't be in Safi and/or Tunis even if those nodes could push to Alexandria. If I had one more, I'd send him to Samarkand because, despite having the largest share in that node, Russia has the only merchant and that means they pull all the output to Astrakhan (which is where I fight them for it because my share there is higher). If I had two more, I'd send a merchant to Kashmir and prevent Vijayangar/Nepal from pushing all that trade to Doab. If I had a three more, I'd send one to Venice, where I'd probably be able to collect ~4 ducats per month. If I had four more I'd send one to California to push trade into Nippon (plus I have Alaska, but not 10 provinces yet). If I had five more, I'd send a merchant to Katsina and push trade to Alexandria instead of Timbuktu (I passively push 1.4 through; if I had a merchant there, it'd be closer to 5 and I'd halve the income Ming/Manchu get there).
I wouldn't even consider Tunis until all of those options were exhausted. And that's assuming I had 100% trade share. I don't. I own every single province in the region and I have a 44% trade share. Transfers from downstream would have the theoretical income I'd get out of that province. No thanks.