so its ok to use the Molex 4-pin cables to power the risers instead of 6-pin cables and reserve those for the cards only ?
Yes, its fine unless its a high power draw GPU ( in your case, the 3060ti's are safer to use with Molex powered risers than the 3070's, so use the 1 remaining pcie slot with a splitter to connect the two 3070 risers to them)
can i use the same cable strand for multiple risers or one riser per cable ?
When using molex and sata, use only one strand per riser ( even if the strand has 3-4 connectors )
finally i did the math for your last mentioned point: 5 cards x 135~w per card (of course they are power limited) = 675 W total | and 75w per riser x 5 risers = 375 W
total power needed 1050 W
are you saying i can Not use a 1000w PSU or even a 1050w PSU for 24x7 mining ??
Your math isnt entirely accurate, risers dont always consume 75w thats generally the rated maximum. Figure out a way to measure accurate total power consumption at the wall where you plug the psu in (buy a power meter or use an household appliance stabaliser etc) for accurate measurement, and decide on PSU wattage accordingly. For your 5 GPU's, the risers should be consuming no more than 35-40W each, so the total should be 280W max for the risers, and 675W for the GPUs, making the total 955W which is dangerously close to the PSU's limit, and will likely cause failure in a few days of running. Also remember PSU is rated at 1000W on 240v, on 120v it can only manage about 750W.
No, every manufacturers cables may be different and its high risk. Stick to known brand PSU's like Corsair, EVGA, Antec, Coolermaster etc and make sure you get Gold rated or above. thier product pages list all the cables they come with.