Keeping gains that you gain with AAS is dependent on your post cycle therapy (pct) and quick recovery, your diet and training, and how far past your genetic potential you are if at all. Low carbs alone will not give you the shredded look nor will it keep you lean, it depends on the full effect of your diet (how much of a surplus or deficit you run and what your essential macros are like). Reducing carbs will only reduce water weight given isocaloric diets. To answer your first question, no, bc the test is not what makes you bigger, the excess calories make you bigger. Testosterone just increase the rate of protein synthesis, calorie partitioning, and recovery aspect. The only gains it will give you with a poor diet is water weight gains which will quickly dissipate after the cessation of the cycle. If you want to get bigger and minimize fat gains you will have to use a smaller calorie surplus and the essential macros to achieve this and carbs could be high or low and won't make any difference if comparing isocaloric diets.
2nd question: gains are gains and don't matter how you get them UNLESS you are going past your genetic potential which seems unlikely here (no offense OP). Ketosis itself will not cut fat bc you could bulk on keto just as easily as cut. The calorie deficit is the number one regulator of weight loss while macros and training will determine if most of the weight loss comes from fat or muscle. In short you can expect to keep all or most of your gains after the cycle if you maintain proper nutrition and training after all is said and done.
