No, thisTthis paper is NOT reliable. A non-exhaustive list of problems in no particular order appears below, but first let's consider their Results:
Figure 2: Yes, they show that BNT162b2 vaccine mRNA enters the cultured cells. However, all their statistical comparisons except for those against the controls are invalid -- see pt. 2 below.
Figure 3: They fail to show that BNT162b2 affects LINE1 mRNA levels. First, the control plots are visually not different from those of the vaccine-treated samples. Second, essentially all their statistical comparisons seem to be invalid -- see pt. 2 below.
Figure 4: Fluorescence always makes pretty pictures, but this figure again seems to fail to show anything useful. First, the LINE1 protein antibody used binds to the LINE1 ORF1 protein, which does not have reverse transcriptase activity, rather than the ORF2 protein, which does contain reverse transcriptase activity. An anti-ORF2 Ab is available, but they did not use it. Also, from Fig. 3 we see no significant difference in LINE1 RNA levels, so LINE1 protein levels would be unlikely to vary. Finally, the statistical comparisons shown are likely to be invalid -- again, see pt. 2 below.
Figure 5: This section claims to detect reverse-transcribed BNT162b2 in DNA preps from buffer-washed cells previously exposed to BNT162b2. Maybe they have, but key controls are missing.
The one piece of evidence shown is successful PCR amplification of a segment of the vaccine sequence. They do show some useful controls, such as lanes for cells not treated with the vaccine, but miss other essential controls such as cells treated with a different mRNA vaccine not containing that amplicon, and DNA from relatively karyotypically and morphologically normal cultured cells treated with BNT162b2, another mRNA vaccine, or nothing. It's all too easy to have a few stray molecules of a DNA fragment present in the lab (like the BNT162b2 PCR amplicon) contaminate other samples and lead to false positives after lots of cycles of amplification. They also provide no evidence any reverse-transcribed BNT162b2 is integrated into the cells' genomic DNA.
As for the paper's significance to BNT162b2 influence on actual normal human cells, there really isn't any because of how abnormal this immortalized liver tumor-derived cell line is in terms of of genomic stability and of gene expression. It's sort of like using Godzilla as a model organism for humans.
The promised non-exhaustive list of problems:
The Zhang et al. (PNAS May 25, 2021 118 (21) e2105968118; https://doi.org/10.1073/pnas.2105968118 ) paper mentioned at the end of your questionin this paper is discussed in my answer to a Medical Sciences StackExchange question: MedicalSciencesQuestionAboutZhangPaper
The summary of my answer to "can the SARS-COV-2 RNA modify our DNA[based on this paper]" is "The short answer is maybe, but rarely, and the whole Covid virus has never been seen to integrate into the cell's DNA intact. Any integration requires "helper" molecules not found in the Covid virus, and only very rarely found in a normal cell."