Manu Samhita does not specify years of the gods. I dont know if you have read the text? ALso, Vedas give no mention of Yuga calculations. Manu Samhita seems to be the original work. Yuga simply means a conjunction or a couple. THe yuga in the surya siddhanta is a yuga of all the planets in aries. These are thought to happen every 108,000 years. A chaturyuga is 4 of these conjunctions, Chatur means 4. thus 432,000 years. So what they are saying is kali yuga of 432,000 years is FOUR yugas already. Its illogical to call a period of 4 yugas as one yuga callled kali.
The yugas of Manu are a union of a rising age and a falling age, thus the union is of these two parts of the cycle. They are divided into 4 parts for sake of convenience, but could be divided into different parts as well asuch as the 27 parts based on the motion of the saptarishis through the nakshatras as mentioned in Brihat Samhita.
The yuga of s urya siddhanta which is based on all the planets joining in Aries is for the purpose of calculatling the mean motions of the planets. If we know that every 108,000 years the planets will be in aries, we can determine their mean placements at any time.
No, Manu Samhita does not say years of the gods. If you look at many translations years of the gods will be in (), thus (years of the gods) because its not literally in the sutra. Its such a part of hindu culture to have such long yugas that they always add year of the god's to the translations. The puranas give excessively long yugas but they are not correct as the puranas were penned between 400 and 700 AD. We do not have the original versions of the puranas and like all hindu literature, they excessive yuga length was inferred due to prevailing thought.
Practically speaking, we are in the electrical age just as Yuktesvar said. If we examine human history the Yuga based on the precession of the equinoxes shows itself working. How we do explain a chnage of the rise and fall of many great cultures in the last few thousand years on a 432,000 year long kali yuga?
The book, 'The Yugas' by Joseph Selbie and David Steinmetz is a book about verifying Sri Yukteswar's take on the Yugas by going into extensive historical evidence to support it. It is a book with about 3 decades of research and is an amazing read.
In the end, if it does nto have practical value, its valueless. If you can make extremely long yugas make sense with the human experience of the past history of man, then use long yugas. Personally, I cannot make any practical use of yugas lasting that long in respect to the actual historical events and i have to enter a world of extreme fantasy that is not supported by any realities past or present.