I do not think that you need to be around to make connections. I have had colleagues who were at office once every blue moon but everyone in the office knew them and invited them everywhere. And I also know of people who sit on opposite desks for years and only have greetings and coffee chit-chats when forced to.
Anyone with good people skills will break any barriers to connect and build a network of influence regardless of their expertise level. Anyone with communication issues or extremely introverted will have more difficulties to connect however, not that they would be doing great in person.
If non-cooperative senior coworkers are stonewalling, then it is a cultural or personal issues that needs to be managed, not the mode of work.
Starting a new job is always stressful, as any major change is difficult for human to adapt and needs time.
> See the countless stories about people holding two jobs at the same time.
This needs to stop, there were few reports, not countless. A few bad apples does not generalize the orchard. Also, if the pay is decent and living costs are affordable, no one needs to hold two jobs. If someone is holding two jobs remotely, it mostly means they are in "bulls!!t" job where they are not utilized to adequate levels and the symptoms of low output is not correctly noticed.