Even with Amex Plat 3X offer, you get 7.5 points per Rs.100 with a Forex charge of 4.13% (3.5%+18% of 3.5%). Even if you assume reward point value at Rs.0.50 per point (Taj Voucher conversion, you can sometime get higher value through Marriott but that required lot of planning and luck), you end up incurring fee of 0.225%.
For Magnus, even after 1Lakh milestone (Till 1 lakh there is no comparison), you get 6 Reward points eq to 4.8 miles for many partners like Vistara/Accor etc. Rs.0.8 is fair value for these points (here also option to get considerable higher value on business class tickets and Accor conversion). This comes with a fee of 2.36% (2% + 18% of 2%). With this you will be getting reward rate of ~1.57%.
Only variable pending above is the exchange rate. In my experience the exchange in Visa is better more often than not. Also the difference in reward rate will never set off any exchange rate difference.
As evident from above calculations, I strongly believe that Magnus make lot more sense for Forex spends vs the Amex Platinum.
Comparison between Vistara Infinite and Magnus becomes lot more subjective. Vistara infinite also has a good rate but need to consider 2 thing for the same. The conversion fee is 3.5% vs 2% of Magnus. Also the reward rate largely depends on value you put on the business ticket. For every 2.5Lakh spend on Infinite, you will end up paying INR 4425 as additional Forex fee and get 4500 less Vistara point vs Magnus. In your view, if that is worth a business class ticket, you can very well go for Vistara infinite. If you can hit the milestones in domestic spends, its prudent to use Vistara infinite for domestic spends and put international spends on Magnus so as to avoid the addition INR 4425 which you are paying in forex fee.
Hope above helps.