„A frontage road (also access road, service road, parallel road, etc.) is a minor road running parallel to a higher-speed more major road, often in an urban setting.“
and
„Do not use the highway= tag for the frontage road. Simply use one of the normal highway=* tags. As the frontage road is less important than the major road, it must also have a lower highway=* tag than the major road“
From my understanding we should be using highway=tertiary for most frontage roads that run along the main highway for significant distances, and in rare cases highway=unclassified. As for oneway, that actually depends on the highway. In many cases bidirectional traffic is allowed, usually to allow local village traffic to go in both directions without crossing the main highway, or worse, traveling in the wrong direction on the main highway. Without looking at actual signage on the ground or the highway DPRs its not possible to state if any arbitrary frontage road should be oneway or not. As for crossing highways, I guess you are referring to when more minor roads are allowed to cross dual carriageway highways? Indian NH design standards do permit this provided the expected cross traffic is low enough, although modern NHs are opting for proper grade separated interchanges.
Its possible there is a connecting road in this instance, since there is a flyover. But its not possible to tell from satellite imagery, it needs someone on the ground to confirm, or at least GPS traces
I will start to change the frontage roads as described in the wiki (tertiary,unclassified).
Since I can not see whether these roads are oneway roads or not, I will not change the mapping in this point.
Yes, the sat images (we can use) do not show clearly whether there are tunnel connection between both NH sides. So I hope, local mappers will help here.